Fix fcitx5 candidate selection commits

[itsXuSt]

Summary

• defer Linux IME candidate selection keys so committed text can take precedence
• prevent deferred candidate keys and delayed composition handling from duplicating input
• track data already forwarded by input events in CompositionHelper

Fixes #6036

Validation

• git diff --cached --check passed before commit
• npm run lint-changes failed before linting changed files because the current oxlint configuration uses reserved JS plugin name eslint
• npm run build failed due to missing existing addon dependencies/types: xterm-wasm-parts, sixel, opentype.js

[itsXuSt]

Validated the updated approach against VS Code's bundled xterm.js on Linux with fcitx5 application preedit disabled. VS Code enables kitty keyboard protocol (useKitty: true), so the candidate-key defer path must still run with kitty enabled. A short keyup fallback delay was also needed so the IME input event can commit the selected candidate before the raw selection key is sent. The verified build preserves Chinese candidate commit for number/space selection in VS Code.