fix: retry rule load after a transient failure

[sarahxsanders]

FROM SIGNALS, REVIEWING MYSELF BEFORE READY

Problem and acceptable goal

Problem this change addresses

scan() lazily assigns rulesPromise = loadRules() on first use and reuses that promise forever. If loadRules() rejects — a transient WASM read failure, a momentarily missing rules directory, or a compile error — the rejected promise stays cached. Every later scan() re-awaits the same rejected promise and throws, so a one-time recoverable failure permanently breaks scanning for the lifetime of the process.

Why the Warlock is the right home for this change:

This is a correctness bug in Warlock's own lazy-load cache inside src/scanner/engine.ts. The faulty caching lives entirely in the engine, so the fix belongs here — pushing retry logic into context-mill or any other consumer would just paper over an engine defect and force every consumer to reimplement the same workaround.

Changes

• scan() now attaches a .catch to the cached loadRules() promise that resets rulesPromise to null and re-throws, so the next scan() retries the load. The success fast-path (compile once, reuse) is unchanged, and it's concurrency-safe: in-flight callers all see the rejection and only the next call triggers a fresh load.
• New src/scanner/__tests__/engine.retry.test.ts forces a one-time readdirSync failure, asserts scan() rejects, then asserts the next scan() recovers. It lives in its own file because the module-level rulesPromise is shared across tests in a file — a fresh file gives a clean module instance so the failure path is actually exercised.

Anti-goal check

• This is not a code-quality or best-practice check
• This does not add policy or enforcement logic
• This does not add phase, tool, or workflow awareness to the engine
• This does not encode wizard-specific assumptions

Test plan

• Existing tests pass (517 passed)
• Build succeeds (npm run build)
• New test covers the transient-failure-then-recovery path
• For new rules: n/a (no rule changes)
• For API changes: n/a (public scan() signature unchanged)

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