test: deterministic test infrastructure

[sderrow]

Empirical soak runs (20× full suite, 15× isolated) surfaced flakey tests, mostly around timing. This PR rebuilds the test infrastructure so timing flakes are structurally impossible rather than individually patched:

• The local project runs on vitest fake timers (auto-advancing tick mode): timing assertions are now exact virtual times, and the project's wall time dropped ~9s → ~1.6s.
• Redis-backed tests assert causality and state (deferred job holds, state polling) instead of wall-clock bounds, which real network time can't honor.
• A test.extend() fixture layer guarantees limiter/connection teardown even when assertions throw — leaked heartbeats were amplifying the flakes — plus custom matchers that are exact under fake time and lower-bound under real time.

Validated with repeated 50-run soaks: zero failures.

[sderrow]

• test: server compatibility matrix — redis 6/7, valkey 8/9 #13
• feat!: remove the callback-style submit() API #12
• refactor: style modernizations and native test assertions #11
• test: deterministic test infrastructure #10 👈 (View in Graphite)
• main

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[sderrow]

Merge activity

• Jul 12, 4:25 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 12, 4:25 AM UTC: @sderrow merged this pull request with Graphite.