perf(emoji): cache caret session state

[JustYannicc]

What changed

• Added an emoji caret session snapshot that captures the frontmost PID, bundle id, focused AX element, and caret rect from the first successful query character.
• Reused the cached caret/bundle payload for subsequent emoji query updates and backspaces while the event target PID remains stable.
• Invalidated the cache on dismiss/reset, PID changes, secure fields, and no-rect AX results.
• Added a focused Swift-backed cache test that runs through the existing node --test 'scripts/test-*.mjs' suite.

Why

The native emoji trigger monitor previously asked NSWorkspace.shared.frontmostApplication and AXCaretQuery.current() for every emitted query update. Typing a multi-character emoji search could therefore repeat frontmost and Accessibility caret work on every character/backspace.

Metrics

Focused stubbed measurement for typing :fire plus one backspace:

Path	Query emits	Frontmost lookups	AX caret queries
Before	5	5	5
After cached rect path	5	1	1

No-rect results intentionally do not cache, so those continue to re-query until a usable rect is available or the session is dismissed.

Validation

• swiftc -O -o /tmp/supercmd-native-check/emoji-trigger-monitor src/native/emoji-trigger-monitor.swift src/native/ax-caret-query.swift src/native/emoji-caret-session-cache.swift -framework AppKit -framework ApplicationServices
• node scripts/test-emoji-caret-session.mjs
• node --test 'scripts/test-*.mjs' (passes; live Electron crash-recovery test is skipped in this environment)
• ./node_modules/.bin/tsc -p tsconfig.main.json
• node scripts/check-i18n.mjs (strict locales pass; pre-existing Italian missing-key warnings remain)
• git diff --check

Notes and risks

• SourceKit LSP sees the Swift files in isolation and reports companion-file symbols as missing; the direct multi-file swiftc compile is clean.
• node scripts/build-native.mjs compiled the Swift helpers, then failed in the existing native addon step because node-addon-api was unavailable under this machine's pnpm/npx wrapper setup.
• The cached path assumes normal CGEvent target PID availability. If the PID is missing, the monitor falls back to one frontmost PID lookup while a cached session is active to avoid reusing state across app switches.

[JustYannicc]

Superseded by the consolidated renderer performance PR #533: #533. Closing this scoped source PR so review stays centralized there.