test(fsrs): add unit tests for FSRS-lite module

[yizhibibao125]

Summary

Add a dedicated test file for the fsrs module (packages/core/src/palace/fsrs.ts), which had no test coverage despite being a core mathematical primitive used by insights, room facts, and pipeline syntheses.

What's covered

• initFsrs — default stability, confirmations seed, last_confirmed validity, explicit now parameter
• score — retrievability range [0,1], decay over time, age_days accuracy, all four status bucket boundaries (hot / warm / cool / archive_candidate), stability passthrough
• reinforce — exact 30% stability growth, confirmations increment, last_confirmed update, pure-function guarantee, compounding over multiple calls
• penalize — stability halving, floor at 1 (including repeated penalizations), immutability of last_confirmed and confirmations, pure-function guarantee
• Combined workflows — reinforce after penalize, dormancy + refresh cycle
• Exported constants — ARCHIVE_THRESHOLD, HOT_THRESHOLD, DEFAULT_INITIAL_STABILITY

Tests

npm run build -w packages/core
node --test packages/core/test/fsrs.test.mjs

## Notes

- Pure test addition — no runtime code changed.
- Style follows the existing suite (`node:test` + `node:assert/strict`, dynamic `import` from `dist/`).