fix Windows CI: add @tapjs/clock devDependency, build tap plugins before tests

[fitzyracing1]

CI on main has had all 4 Windows jobs red since 2026-04-20. Three separate problems, each hidden behind the one before it:

1. .taprc loads the @tapjs/clock plugin, but the package isn't in devDependencies. On Windows tap can't resolve it from the auto-installed .tap/plugins and aborts with "'@tapjs/clock' does not appear to be a tap plugin". Added @tapjs/clock@^3.3.10, which matches the peer @tapjs/core@4.5.8 that tap 21.7.4 uses.
2. On Windows the first tap run, which builds the plugin set, exits 1 even when every test passes. Later runs exit 0. pretest now runs tap build after prepare, so the test run starts with plugins already built.
3. Coverage was 99.96% on Windows: the autopurge re-arm path from 0b0a77e (index.ts:1641) is only reached through timer drift. Added test/ttl-autopurge-rearm.ts, which uses a frozen perf clock so the timer fires before the entry is stale and gets re-armed every time.

Verified: CI on my fork is green on all 8 jobs (ubuntu, macOS, Windows bash, Windows powershell × Node 24 and 25) at this branch's head commit 5f4a2e3: https://github.com/fitzyracing1/node-lru-cache/actions/runs/38201310520. The full suite also passes locally on Linux.