Architecture: Make FileStore's JSON serialization contract explicit

[jonbaldie]

Summary

• Constrain FileStore<T> to the existing JsonValue type and document its JSON persistence contract.
• Keep QueueStore<T> and MemoryStore<T> generic; update JSON persistence fixtures to use JSON-compatible types.
• Add compile-time rejection coverage for FileStore<Date>, nested JSON close/reload coverage, and exact JSON-lines format assertions.

Verification

• deno check main.ts tests/*.ts fuzz/deep_fuzz.ts
• deno test --allow-read --allow-write --allow-net --allow-env --allow-run tests/persist_test.ts tests/manager_test.ts (87 passed)
• deno run --allow-read --allow-write --allow-net --allow-env --allow-run fuzz/deep_fuzz.ts (192 checks)
• npm run quality:production
• Full suite passed once (373 tests). A later local rerun failed only the three compiled-binary startup tests with empty child output; the targeted and production checks pass. Awaiting the PR CI run.

Closes #158