fix(signing): structured-headers parser + bucketed replay store (#581, #582)

[bokelley]

Closes #581 (parser swap). Closes #582 (replay store).

#581 — swap to structured-headers

Hand-rolled Signature-Input / Signature / Content-Digest parsers replaced with the maintained structured-headers library (RFC 8941 / RFC 9651).

• ~90 lines of bespoke state-machine code removed (src/lib/signing/parser.ts + content-digest.ts).
• AdCP-profile checks kept as thin typed wrappers: required-param presence, tag match, alg allowlist, quoted-string typing for nonce/keyid/alg/tag, integer typing for created/expires.
• Signature byte-sequence values stay base64url-tolerant — we normalise -/_ → +// only inside :…: delimiters before handing to the strict SF parser.
• Content-Digest keeps a regex fallback so a malformed filler member (e.g. truncated sha-512) does not mask the sha-256 entry we actually verify against.
• signatureParamsValue is now serializeInnerList(entry) instead of raw substring splicing — verified byte-identical on the reordered-params review-hardening vector.
• Escape decoding now follows RFC 8941 §3.3.3 (the old parser stored the literal \" escape sequence, the library decodes to "). The one escape-handling test assertion updated to match correct behaviour.

#582 — time-bucketed replay store

InMemoryReplayStore now groups entries by floor(expiresAt / bucketSizeSeconds) (default 60s). Whole buckets evict in one step when their latest expiry has passed, eliminating the per-call O(N) filter sweep that turned a near-cap keyid into a quadratic DoS target.

• has() / insert() / isCapHit() are O(1) amortized per keyid regardless of entries-per-keyid.
• Prune runs at most once per bucket tick per keyid.
• Default maxEntriesPerKeyid: 1,000,000 → 100,000 (still ample; new InMemoryReplayStore({ maxEntriesPerKeyid }) overrides).
• ReplayStore interface unchanged.

Test plan

• All 28 RFC 9421 conformance vectors pass
• test/request-signing-review-fixes.test.js passes (with 2 assertion tweaks for the correct RFC 8941 escape-decoding + library-surfaced parse-error messages; intent preserved, + a new created=1.5 integer-rejection assertion)
• test/request-signing-e2e.test.js passes
• New test/request-signing-replay-perf.test.js asserts sub-linear (< 4×) growth of has() / insert() latency when entries-per-keyid grows 50×, plus a bucket-eviction correctness check
• Full npm test green (3601 passing)
• Changesets added (two patch entries)

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