This is a partial port of a technique I am adopting in nab, I have been
following @konstin's lead in
astral-sh/pubgrub#75 and extending the approach
into more places (notatallshaw/nab#357,
notatallshaw/nab#590,
notatallshaw/nab#592,
notatallshaw/nab#604).

This speeds up `uv lock` with `requires-python >= 3.8` over boto3 +
awscli by ~20% wall time, and the largest accumulated range in the run
drops from 287 segments to 2.

`add_unavailable_version` records one singleton incompatibility per
rejected version, so consecutive rejections never coalesce and the
accumulated range grows a hole per version. This widens each singleton
to the gap between its neighbors in the known versions, the same
interval decided-version dependencies use.

The win needs long runs of rejected versions. The main source is a
Python floor below the ecosystem's support boundary: boto3 and awscli
dropped 3.8 in April 2025 and now require >= 3.10, so a 3.8 floor
rejects hundreds of releases in one run, while a 3.10 floor rejects
almost nothing (14 rounds, 2 segments either way) and the change is a
no-op.

The first commit keys the pre-release hint on the requesting requirement
instead of the derived set's bounds: widened bounds land on whatever the
registry lists next, and a pre-release neighbor used to read as a
pre-release marker nobody wrote. Rejection widening makes that routine,
so the fix leads the series. The second commit merges unavailable
versions that are causes of the same derivation, and the third is the
widening itself. The fourth keeps the subsequent-index hint, which was
suppressed whenever the set covered all versions; after widening that
also covers "the index listed versions and none of them worked", which
is the case the hint is for.

The last two commits are #20809, merged in here so the error messages
land as one change. With the widening, uv no longer asks for a version
in a gap it created, so the `no_versions` incompatibility behind "only
the following versions of X are available" is no longer created there,
and steps that cross a gap stop following from the premises next to
them. The fifth commit adds that clause back while the tree is reduced,
on whichever cause rules the package out, or on the step itself when
neither does. The sixth reports the one version a package has as that
version rather than as a range over it, so a rejected `sphinx 7.2.6`
reads `sphinx==7.2.6` and not `sphinx>=7.2.6`. Neither touches the
solver.

<details>
<summary>MRE</summary>

```toml
[project]
name = "mre"
version = "0.1.0"
requires-python = ">=3.8"
dependencies = ["boto3", "awscli"]
```

Max accumulated segments (287 on main, 2 with this PR; `uv.lock`
identical):

```console
$ uv lock --exclude-newer 2026-07-01 -v 2>&1 | grep 'Searching for a compatible version' | awk -F'|' '{print NF}' | sort -n | tail -1
```

Wall (run without `-v`, which prints the accumulated ranges and
dominates the time):

```console
$ hyperfine --warmup 3 --min-runs 20 --prepare 'rm -f uv.lock' 'uv lock --exclude-newer 2026-07-01'
```

</details>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

When dependency resolution rejects package versions one at a time, its
version sets can accumulate a hole for every rejection. This PR lets
callers widen a selected version across gaps between known versions when
recording its dependencies, allowing adjacent incompatibilities to merge
into contiguous ranges and keeping subsequent range operations small.

The built-in resolver preserves the existing singleton behavior;
widening is opt-in for callers with a complete, fixed version list.
`Ranges::narrow_versions` provides the display-oriented inverse for
presenting widened ranges in terms of known versions without hiding
unbounded coverage.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>