Fix GlobalKeyMap matching for keyboards with layout-dependent charactersIgnoringModifiers

[TankTechnology]

Problem

On keyboards where the same physical key reports different charactersIgnoringModifiers values depending on input method state (e.g., Chinese/Pinyin layouts where the backtick key returns 0xb7 in one mode and 0x60 in another), GlobalKeyMap key bindings become unrecognized after text input.

Reproduction

1. Configure a GlobalKeyMap binding for Cmd+` (recorded when the keyboard returns char=0xb7)
2. Open iTerm2, create two split panes
3. Type any character in one pane (changes input method state)
4. Press Cmd+` — the binding is not recognized

The stored binding key is 0xb7-0x100000-0x32 but the keystroke from the event serializes as 0x60-0x100000-0x32 because charactersIgnoringModifiers now returns 0x60.

Root Cause

In iTermKeystroke.keyInBindingDictionary:, the normal-path fallbacks all depend on the character field (from charactersIgnoringModifiers):

1. serialized — 0x<char>-0x<mods>-0x<keycode> ✗ (char differs)
2. legacySerialized — 0x<char>-0x<mods> ✗ (char differs)
3. modifiedSerialized — :0x<modChar>:0x<mods> ✗ (char differs)
4. Slow legacy iteration — same problem

None of these match when the character field varies.

Fix

Add a final fallback using portableSerialized (*-0x<mods>-0x<keycode>), which matches on virtual key code and modifiers only — the same strategy already used when LanguageAgnosticKeyBindings is enabled. This is a 15-line addition that only activates when all existing matching strategies have failed, so it's fully backward-compatible.

Commits

1. Test reproduction — Python script that 1:1 mirrors the keyInBindingDictionary: matching logic, plus a Swift XCTest
2. Fix — Add portableSerialized fallback in the normal matching path

Testing

Run tests/reproduce_keystroke_bug.py to verify the fix:

$ python3 tests/reproduce_keystroke_bug.py
BEFORE fix: NO MATCH — BUG REPRODUCED
AFTER  fix: MATCH

The Swift XCTest in ModernTests/iTermKeystrokeBindingTests.swift covers the same scenarios and can be run with Xcode.

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