test(match): add direct unit tests for requestKey, responseKey, methodOf

[protosphinx]

Why

requestKey and responseKey are exported from src/match.ts and
re-exported via src/index.ts. They form the backbone of the
diffTranscripts logic: every pair comparison goes through them. Yet
they had no dedicated unit tests - only indirect coverage from
test/diff.test.ts exercising them at the diffTranscripts level.
methodOf is also exported but untested. Adding focused tests closes
this gap and makes regressions in these functions immediately
diagnosable rather than surfacing as mysterious diff failures.

What

• New file test/match.keys.test.ts with 21 tests across three suites.
• methodOf: returns method string for requests and notifications,
  undefined for responses.
• requestKey: stable across param key order and request ids, strips
  volatile keys (timestamp, ts, requestId), differs on method and
  non-volatile params, handles no-params and no-method messages.
• responseKey: stable across response ids and result key order,
  differs on result values and error codes, distinguishes result from
  error, strips volatile keys from results.

Tests

• All 21 new tests in test/match.keys.test.ts.
• Full suite: 111 passing (12 test files). Was 90 passing (11 files)
  before this PR.

Self-merge gate

• all CI checks pass
• LOC delta < 250 (added + removed)
• no public-API surface change
• no runtime-dependency additions
• no workflow file changes
• tests added or extended

---

Generated by Claude Code

[protosphinx]

auto-deferred for human review: CodeQL SARIF status check stuck in queued state after extended wait

The primary CI results:

• test: success
• Analyze (actions) (the CodeQL scan job itself): success

The CodeQL check (SARIF result processing) has been in queued status for more than 10 minutes without resolving. Self-merge gate requires all checks to be non-pending. Safe to merge manually once CodeQL resolves or is confirmed neutral.

---

Generated by Claude Code