feat: auto-clean Story.on() handlers on restart (#128)

[rohal12]

Summary

• Add runtime phase tracking (trackRuntimeUnsub, enterRuntimePhase) to store.ts — handlers registered via Story.on() after executeStoryInit() are automatically unsubscribed when restart() runs
• Wire all 5 event branches in Story.on() (navigate, beforerestart, storyinit, actionsChanged, variableChanged) to call trackRuntimeUnsub
• Call enterRuntimePhase() before executeStoryInit() in both boot (index.tsx) and restart (store.ts) paths

Fixes #128

Design

Handlers registered during :storystartup (startup phase) survive restarts. Handlers registered from executeStoryInit() onwards (runtime phase) are auto-cleaned on restart, after fireBeforeRestart() fires but before state resets. This prevents duplicate handler accumulation when host apps re-register during storyinit.

Test plan

• trackRuntimeUnsub is a no-op before enterRuntimePhase
• Tracked unsubs are called on restart
• Startup-phase handlers survive restart
• Unsubs cleared after restart (not called twice)
• Navigate handler cleaned on restart via Story.on()
• beforerestart handler fires before being cleaned
• No duplicate handlers after multiple restart cycles
• Manual unsub + double-call is safe

🤖 Generated with Claude Code

[github-actions]

Release preview: merging this PR will publish v0.40.0 (minor bump from v0.39.1)

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	77.71%	2751 / 3540
🔵	Statements	76.8%	3013 / 3923
🔵	Functions	68.79%	507 / 737
🔵	Branches	73.96%	1506 / 2036

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/index.tsx	0%	0%	0%	0%	21-219
src/store.ts	80.55%	72.72%	73.19%	82.31%	90, 124, 348-349, 412, 421-424, 455-460, 552, 607, 630-633, 647, 651-654, 666-738
src/story-api.ts	26.21%	14.06%	25%	27.09%	167-176, 186-190, 198-282, 295-373, 402-453, 480-485, 512-547

Generated in workflow #199 for commit f3ea200 by the Vitest Coverage Report Action