fix: use unique IDs to prevent instance collisions across bundles

[layershifter]

Summary

• Replaces the module-scoped _lastId counter with crypto.getRandomValues() to generate unique instance IDs
• When multiple bundles include keyborg on the same page, they each had their own _lastId starting at 0, causing ID collisions in the shared window.__keyborg.refs map
• With random IDs, each instance is guaranteed unique regardless of how many bundles coexist
• Uses crypto.getRandomValues() (not crypto.randomUUID()) to support Edge 84+, Firefox 75+, Chrome 84+, Safari 14.1+

Closes #99
Supersedes #75 and #104

Test plan

• npm run build passes
• npm run lint passes
• All existing Playwright tests pass (10/10)

🤖 Generated with Claude Code

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
keyborg All exports	6.991 kB 2.077 kB	7.121 kB 2.155 kB	130 B 78 B
keyborg createKeyborg() & disposeKeyborg()	6.755 kB 2.028 kB	6.885 kB 2.105 kB	130 B 77 B

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
keyborg KEYBORG_FOCUSIN constant	64 B 80 B

_{🤖 This report was generated against 4658ddcd049caa75a25c67ed801c1d19229f5353}