perf: drop unused code paths (event.details alias, KeyborgProps, IE9 probe)

[layershifter]

Summary

Three independent trims that remove code paths none of keyborg's known consumers exercise. Upstreams the changes microsoft/tabster#539 currently applies via patch-package against keyborg@2.14.0's installed bundle.

The commit is split into three commits for clean review and bisection:

1. event.details alias on KeyborgFocusInEvent — the @deprecated alias of event.detail. Tabster reads e.detail exclusively (verified across src/State/FocusedElement.ts and the rest of the codebase). Drops the field on the interface plus the per-dispatch event.details = details assignment in setupFocusEvent.

2. KeyborgProps (triggerKeys / dismissKeys) + dismiss timer machinery — KeyborgProps was never re-exported from src/index.mts and no known consumer passes props to createKeyborg. Removes the interface, the triggerKeys/dismissKeys Sets, shouldDismiss/scheduleDismiss/dismissTimer/_dismissTimeout, and collapses onKeyDown to a single trigger guard.

3. canOverrideNativeFocus runtime probe + _canOverrideNativeFocus flag — the probe detected pre-IE9 environments where reassigning HTMLElement.prototype.focus was ignored. Every browser keyborg currently supports honours the override, so the gated details.isFocusedProgrammatically write becomes unconditional. Semantically identical when the override works, which is the only case we still ship for.

Public API impact

• KeyborgFocusInEvent interface no longer carries details?: KeyborgFocusInEventDetails. The field has been marked @deprecated for several releases; consumers should use event.detail.
• createKeyborg(win, props?) → createKeyborg(win). KeyborgProps type is removed. Not previously re-exported, so the only breakage is callers passing the second argument (the type narrowing already silently ignored unknown shapes — passing undefined continues to work since the parameter is gone entirely).
• No runtime contract changes on __keyborg.core or __keyborg.refs.

Worth treating as a minor or major bump depending on how the maintainers want to handle the dropped details field.

Bundle deltas (monosize)

                                    Before        After          Δ
All exports                         3.974 kB min  3.363 kB min   −611 B (−15.4%)
                                    1.627 kB gz   1.409 kB gz    −218 B (−13.4%)

createKeyborg + disposeKeyborg      3.806 kB min  3.195 kB min   −611 B (−16.1%)
                                    1.586 kB gz   1.368 kB gz    −218 B (−13.7%)

KEYBORG_FOCUSIN constant            64 B / 80 B   64 B / 80 B    unchanged

Matches microsoft/tabster#539's measurement of −590 B on keyborg's slice of the Tabster bundle within ~20 B of slack.

Tests

Existing Playwright suite (14 tests covering focus behavior, focus-in events, shadow DOM nesting, and cross-version interop) passes unchanged.

Test plan

• yarn build (CJS + ESM, types)
• yarn lint
• yarn format
• yarn test (14/14)
• yarn bundle-size
• Reviewer: decide on semver level (minor with deprecation removal call-out vs. major)

🤖 Generated with Claude Code

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
keyborg All exports	3.974 kB 1.627 kB	3.363 kB 1.409 kB	-611 B -218 B
keyborg createKeyborg() & disposeKeyborg()	3.806 kB 1.586 kB	3.195 kB 1.368 kB	-611 B -218 B

Unchanged fixtures

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

_{🤖 This report was generated against dfd7fe32327b37d8746c44aef6cebeda1ecc2397}

[layershifter]

Splitting into three focused PRs, one per trim, for independent review.