perf: refactor Keyborg classes to closure-based factories

[layershifter]

Summary

• Rewrites src/Keyborg.mts to use factory functions instead of classes. Per-instance state lives in closure variables, which the minifier can mangle; class property names like this._triggerKeys cannot be mangled.
• Public API unchanged. Keyborg becomes an exported interface (it was already only re-exported as a type from index.mts). createKeyborg, disposeKeyborg, and the KeyborgCallback type are unchanged.
• WindowWithKeyborg.__keyborg retains its outer { core, refs } shape for any external consumer (e.g., Tabster) that reads it. The internal core handle shape changes: it is now { dispose, getNavigating, setNavigating } instead of a KeyborgCore class instance with a getter/setter for isNavigatingWithKeyboard.

Notes for reviewers

• The internal core.isNavigatingWithKeyboard getter/setter is gone. If any consumer reads window.__keyborg.core.isNavigatingWithKeyboard directly, that is an undocumented leak and would break here — please flag if aware of any such consumer.
• The _id values assigned to instances no longer interleave with core counters. This only shows up in the dev-mode "Keyborg instance <id> is being disposed incorrectly" console.error string.

🤖 Generated with Claude Code

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
keyborg All exports	6.606 kB 1.957 kB	4.893 kB 1.746 kB	-1.713 kB -211 B
keyborg createKeyborg() & disposeKeyborg()	6.37 kB 1.904 kB	4.657 kB 1.693 kB	-1.713 kB -211 B

Unchanged fixtures

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

_{🤖 This report was generated against f823bc9619ff7df43dc54c92e66889aa3bed90bf}