Cleanup: trim draft-20 comments, consolidate tests (no behaviour change)

[floatdrop]

A cleanup pass over the draft-20 compliance work (#95–#100), which added about 17k lines: 11k of tests and 6k of production code, 38% of that comments. There is no behaviour or API change:

• production .go files change only in comments, plus two gofmt field re-alignments;
• tests are consolidated, with no test dropped and no assertion weakened.

Net result: 196 files changed, about 4,400 fewer lines.

Production comments (aggressive trim, as decided)

• 7ec6c2a message, wire, uri
• ea63596 session
• 2b6805d relay: handlers, registry, cache, discovery

What was kept:

• exported doc comments (contract, errors, concurrency);
• one-line § citations;
• one line wherever a spec interpretation or deviation, a lock-ordering or concurrency invariant, or a non-obvious ordering reason would otherwise be lost;
• //nolint reasons.

History narratives, restatements of the code, and long or repeated spec quotations were removed.

Production Go: 2,849 lines removed, 1,268 added.

Tests

• 6260ea3 session:
  • one helpers_test.go (a single handshake helper, one close-code recorder, shared stubs);
  • one-fix files folded into topical ones;
  • near-copies table-driven.
• 9abdcc4 message, 2a1cef5 wire, 2ea4f6e registry, c2e4437 cache, 8338605 discovery: shared helpers, folded files, slices.Equal / generic receive helpers.
• 80352cc, d99dc81 relay:
  • about 156 helpers reduced to a small set in harness_test.go, helpers_test.go and helpers_fetch_test.go;
  • 12 topical files absorb the one-fix files;
  • the auth-denial, token and cache tests are table-driven.

Tests: 9,995 lines removed, 7,184 added, and 209 test files down to 185.

Nothing lost. I ran the full suite on the base and on this branch:

• 1,750 executed tests and subtests before, 1,754 after;
• the 20 top-level names that disappeared each became a case of a table test;
• no timeouts loosened, and t.Parallel() kept.

A few helpers are stricter than before: Close errors and failed accepts now fail the test.

Review follow-ups (c41519d)

A moqt-reviewer pass over the whole branch found no wire-behaviour change and no weakened test. The comment fixes it prompted:

• Restored:
  • the "Assumption:" on RegisterInboundTrack (which Track Properties a shared alias keeps);
  • two exported contracts: OpenPublish callers allocate the alias, and AcceptPublish's refusal codes;
  • four invariants the code doesn't show.
• Citations aligned:
  • omitted FORWARD → §10.2.18;
  • multi-publisher dedup → §9.3;
  • fetch order → §10.13;
  • one response per SUBSCRIBE → §5.1;
  • the §5.1.5 quote corrected.
• Marked as a choice: §11.4.3's next-Object rule, where the relay uses only "one greater than the previous Object".

Not in this PR

• Next-Object tests (§11.4.3). The relay reopens a downstream stream at any Object ID gap, even when the upstream skipped IDs on the same stream, which §11.4.3 allows forwarding. That's valid but wasteful; changing it is a behaviour change for a separate branch.
• Long test files. Four older test files are still over 800 lines (session_pubsub, handler_fanout, cross_relay, datastream_object). They were trimmed but not split.
• A weak older test. TestTrackStatus_ReplyForKnownTrack passes vacuously if its FETCH is refused. This predates the cleanup.

Testing

• go build, go vet, golangci-lint: clean.
• go test ./... and -race on session and relay: pass.
• make bench-quick: allocs/op at the baseline.
• make interop-loopback: 6/6 on raw QUIC and on WebTransport.

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