The tunnel proxy strips Cookie / Authorization inbound and Set-Cookie
outbound by design, so a tunnel exposes PUBLIC content only — no
bearer/DPoP access, and no cookie-session flows (interactive OIDC
login can never complete) through a tunnelled pod.

This adds the opt-in: `{ type: 'register', name, passthrough: true }`.
When the registering client opts in, the proxy forwards Cookie and
Authorization to the tunnel client and Set-Cookie back to the
visitor. Off by default; the ack echoes the applied mode so the
client knows what the relay actually did.

Security shape (documented in the protocol header):
  - Opt-in is per-tunnel and owner-asserted — forwarding means
    visitor credentials bound for the tunnelled service are handed to
    the registered client, which is safe exactly when the registrant
    owns that service (the normal my-pod-through-my-relay case).
  - Strict `=== true` so a truthy-but-wrong value ('true', 1) can't
    silently enable credential forwarding.
  - Proxy-Authorization is ALWAYS stripped — it is addressed to the
    relay, never to the tunnelled service.
  - Set-Cookie array values survive JSON serialization and Fastify
    emits one header per entry.

Known limitation, per the issue: passthrough alone doesn't make
path-based login flows fully work (/tunnel/<name>/ cookie-Path and
redirect rewriting — the reverse-proxy-at-a-subpath problem). It
delivers authenticated API access today and is the prerequisite for
the subdomain-routing endgame.

Tests (4 new, including pinning the previously-untested default):
default strips both ways, passthrough forwards all three headers
(multi-value Set-Cookie included), Proxy-Authorization never
forwarded, non-boolean opt-in rejected. 952/952 passing.

Closes #530.

…feature-detect

Copilot: get('set-cookie') behaviour varies by undici version and may
expose only one value; Headers#getSetCookie() (Node ≥18.15) is the
supported multi-value API. Feature-detect: assert the full cookie
pair on the real API, degrade to a single-cookie assertion on older
runtimes (engines floor bump tracked in #541). Non-flaky on every
Node the engines field admits.

… available

Copilot: on some undici versions get('set-cookie') returns null even
when Set-Cookie headers ARE present (only readable via getSetCookie),
making the default-strip assertion vacuously pass — a false positive
that could mask a regression in the security default. Feature-detect:
assert getSetCookie() returns [] when the API exists, fall back to
the null check otherwise. Mirror of the pass-1 fix, applied to the
strip side.

…rough mode

Copilot found a real session-hijack vector. oidc-provider sets the
relay's _session / _interaction cookies with path=/, so a browser
attaches them to /tunnel/... requests too. With passthrough on, the
proxy forwarded the FULL Cookie header — handing a visitor's RELAY
session cookies to the tunnel client, which could replay them against
the relay's /idp/* endpoints (session hijack / cross-app credential
leak). The tunnel owner opted in to receive credentials for THEIR
service, not the relay's own session.

Fix: even in passthrough mode, strip the relay's own IdP cookies
(_session*, _interaction* — matched at a / boundary so a
tunnelled service's unrelated cookie like  survives)
from the forwarded Cookie header. The visitor's cookies bound for the
tunnelled service pass through; the relay's do not. Defense-in-depth
that holds regardless of cookie-path config or subdomain routing.

Also: corrected the protocol-header ack example from
 to  (the relay echoes
the applied mode — Copilot's second comment).

New test: visitor carries both an app cookie and the relay's
_session/_session.sig/_interaction cookies; only the app cookie is
forwarded. 12/12 tunnel, full suite green.

…eaders

Copilot: header values can be string | string[] in Node/Fastify
(folded duplicate Cookie headers). The helper returned '' for a
non-string input, which would drop ALL cookies in passthrough mode if
an array ever arrives. Normalize array → join with '; ' before
filtering. Defensive (Node usually folds Cookie to a string) but
cheap.