feat(flterm): normalize layout-aware keyboard input

[adonm]

Summary

• normalize physical key, action, text, consumed modifiers, lock/side state, composition state, and active-layout unshifted codepoint before terminal encoding
• add Linux, macOS, and Windows native metadata companions with a documented Flutter fallback for mobile/web
• keep dead keys and IME preedit on the text-input path and suppress paired release leakage
• add side-aware macOS TerminalConfig.optionAsAlt

Why

Flutter's public KeyEvent does not expose consumed modifiers or the active layout's unmodified translation. Capturing platform metadata before framework normalization keeps platform translation separate from terminal protocol encoding and covers non-US layouts, AltGr/Option, dead keys, lock state, repeats, and releases.

July 2026 update: reworked onto current main

Rebased after #143/#144 and reworked for the controller/view input pipeline:

• KeyboardInputAdapter now normalizes through KeyboardEventNormalizer; the previous controller-internal encoder path is gone.
• Upstream's unshifted static map, consumed-modifier fallback, and IME routing now feed through the normalizer, so the native companions only override where they have real metadata.
• Virtual-modifier exclusion and the consumed-ctrl strip are preserved when terminal mods are assembled.
• TerminalConfig.optionAsAlt uses the libghostty OptionAsAlt config enum; TerminalController accepts an optional keyEventNormalizer.

Verification

• Flterm suite with source-compiled native assets (Zig 0.16): 983 passed, including normalizer, metadata-store, keyboard-event, platform-map, and config tests.
• dart format and dart analyze clean (only 2 pre-existing info lints under tool/benchmarks).
• Native companions compile/register for Linux (GTK), macOS (Swift), and Windows (C API).

Closes #70.