Add leftchar validation after the main decode loop in a2b_base64
(strict_mode only) and a2b_base32 (always). Fix existing test data
that incidentally had non-zero padding bits to use characters with
zero trailing bits while preserving the same decoded output.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Gate non-zero padding bits rejection behind a new canonical= keyword
argument independent of strict_mode, per discussion on pythongh-146311.

Per RFC 4648 section 3.5 ("Canonical Encoding"), decoders MAY reject
encodings where pad bits are not zero. The new canonical=True flag
enables this check for a2b_base64, a2b_base32, a2b_base85, and
a2b_ascii85.

For base85/ascii85, the canonical check also rejects single-character
final groups (never produced by a conforming encoder) and verifies
that partial group encodings match what the encoder would produce.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The _Py_ID(canonical) identifier used by the clinic-generated
argument parsing code needs to be registered in the global strings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Replace the re-encode-and-compare loops with a quotient comparison:
two divisions by 85**n_pad tell us whether the decoded uint32 and
the zero-padded output bytes share the same leading base-85 digits.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Test non-canonical rejection for all partial group sizes (2/3/4 chars)
- Test digit-0 1-char group for ascii85 (exercises chunk_len==0 guard)
- Test boundary byte values (\x00, \xff) at each group size

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Round-trip tests: encoder always produces canonical output (base64,
  base32, base85, ascii85)
- Uniqueness tests: for base85/ascii85 partial groups, sweep all 85
  last-digit values and verify exactly one decodes to the original
  payload with canonical=True

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Per the PLRM spec (section 3.13.3), a final partial 5-tuple containing
only one character is an encoding violation. Move this check outside
the `canonical=` guard so it is always enforced.

Also change chunk_len and i from Py_ssize_t to int per review feedback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

When canonical=True, reject '!!!!!' (five zero digits) in favor of
the 'z' abbreviation. The PLRM spec defines 'z' as the representation
for all-zero groups, so '!!!!!' is a non-canonical encoding.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Document that single-character final groups are always rejected
- Add versionchanged:: next markers for the behavioral change
- Update canonical= description for ascii85 to mention z-abbreviation
- Update canonical= description for base85

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…atsnew

- Reorder versionchanged param lists alphabetically in base64.rst and
  binascii.rst (canonical, ignorechars, padded).
- Restore "two to five" in a2b_ascii85/a2b_base85 partial-group text;
  partial finals are 2-4 chars but a complete final group is 5.
- Drop redundant versionchanged blocks from a2b_ascii85/a2b_base85
  since both functions are new in 3.15.
- Add Misc/NEWS.d entry and Doc/whatsnew/3.15.rst bullets covering the
  new canonical kwarg and the unconditional 1-char rejection.

The PLRM (Adobe Ascii85) and the various base85 alphabets do not define
a "canonical" form. Where the underlying spec doesn't ground the term,
the doc text now says explicitly that "canonical" here means "the
encoding the corresponding b2a_* function would produce."

Base16/32/64 entries are unchanged: RFC 4648 grounds the term there.