feat: add beforerestart event hook

[rohal12]

Summary

• Adds a beforerestart event that fires at the top of restart() before any state reset (variables, PRNG, session, etc.)
• Host applications can use Story.on('beforerestart', cb) to clean up persistent state (e.g. sessionStorage, IndexedDB) while pre-restart variables are still accessible
• Follows the existing onStoryInit callback registry pattern

Closes #124

Test plan

• Callback fires on restart
• Fires before storyinit (ordering test)
• Pre-restart variables readable inside callback
• Fires on every restart
• Unsubscribe stops future callbacks
• All 1024 existing tests pass
• TypeScript compiles clean

🤖 Generated with Claude Code

[github-actions]

Release preview: merging this PR will publish v0.39.0 (minor bump from v0.38.1)

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	77.45%	2721 / 3513
🔵	Statements	76.55%	2982 / 3895
🔵	Functions	68.16%	499 / 732
🔵	Branches	73.76%	1499 / 2032

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/store.ts	79.47%	71.57%	71.73%	81.29%	90, 124, 313-314, 377, 386-389, 420-425, 517, 554, 577-580, 594, 598-601, 613-685
src/story-api.ts	18.83%	9.37%	21.05%	19.31%	162-277, 290-368, 373-378, 391-438, 465-470, 497-532

Generated in workflow #195 for commit fe0866d by the Vitest Coverage Report Action