feat: centralized event emitter with save/load/navigate hooks (#129)

[rohal12]

Summary

• Adds src/event-emitter.ts — a typed centralized event emitter replacing scattered listener arrays across store.ts, action-registry.ts, and story-api.ts
• Adds 6 new lifecycle hooks: beforesave, aftersave, beforeload, afterload, beforenavigate, afternavigate — enabling host apps to inject behavior around these operations without monkey-patching
• Simplifies Story.on() to a type-safe thin wrapper delegating to the emitter

BREAKING CHANGE: Story.on('navigate') removed — use Story.on('afternavigate') (same callback signature). afternavigate only fires from explicit navigation (navigate(), goBack(), goForward()), not from loadFromPayload().

Closes #129

Test plan

• 9 unit tests for the emitter module (on/emit/unsub/order/unknown events/mid-emit safety)
• 3 tests for beforesave hooks (argument passing, variable injection, undefined slot)
• 8 tests for beforenavigate/afternavigate (navigate/goBack/goForward/loadFromPayload exclusion/invalid passage)
• 2 tests for beforeload/afterload (ordering, state restoration)
• 2 auto-cleanup integration tests (runtime hooks cleaned on restart, startup hooks survive)
• Existing event migration parity (storyinit, beforerestart, actionsChanged, variableChanged)
• Full suite: 1059 tests pass, tsc --noEmit clean

🤖 Generated with Claude Code

[github-actions]

Release preview: merging this PR will publish v0.41.0 (minor bump from v0.40.1)

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	78.32%	2776 / 3544
🔵	Statements	77.32%	3035 / 3925
🔵	Functions	68.67%	502 / 731
🔵	Branches	74.36%	1514 / 2036

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/action-registry.ts	100%	100%	100%	100%
src/event-emitter.ts	100%	100%	100%	100%
src/index.tsx	0%	0%	0%	0%	22-220
src/store.ts	80.61%	72.72%	70.78%	82.6%	91, 125, 310-311, 374, 383-386, 420-425, 531, 586, 612-615, 629, 633-636, 648-720
src/story-api.ts	30.4%	16.07%	25%	31.88%	76, 181-204, 212-296, 309-387, 403-420, 447-452, 479-514

Generated in workflow #203 for commit 5a64ef6 by the Vitest Coverage Report Action