Add configurable physical-key input-source bindings

[tonatoz]

Summary

 Input-source bindings
-  Left Command and Right Command only
+  Two required rows and up to three optional rows
+  Physical character, function, navigation, editing, Command, Option, and Control keys
+  Native configuration window: Record Key, Apply, Cancel, Restore Standard Keys

• Keep the two Command bindings as defaults and migrate legacy preferences.
• Suppress assigned ordinary keys and their repeats. Preserve modifier-first shortcuts.
• Switch modifier bindings after a solitary release. Keyboard, mouse, and scroll activity cancel the candidate.
• Retain unavailable input-source IDs and pass their keys through until the source returns.
• Commit window edits only with Apply. Disable quick source selection while the window is open.
• Update the README, contributor guidance, and behavioral tests.

Evidence

• Before: Regression checks failed for intrinsic Fn flags on function/navigation keys and missed releases.
  After: Checks pass for intrinsic Fn flags, physical Fn shortcuts, fresh presses after missed releases, and event-tap recovery.
• Before: The app supported only two Command bindings.
  After: Native AppKit smoke checks covered five bindings, recording, duplicate rejection, Apply/Cancel, reset, unavailable sources, and persistence after restart.
• Before: An ordinary unassigned A inserts text.
  After: Assigned A and its repeats insert no text. Command-A still selects all, and normal C replaces the selection.
• make test and make lint passed.
• swift build --build-system native passed. The default backend failed with Unknown error parsing property list in this environment.
• make app -o build BIN=.build/debug/2cmd and codesign --verify --strict dist/2cmd.app passed.
• Code and specification reviews found no remaining issues after fixes.

Merge Danger

Door: one-way

Migration removes legacy preference keys after saving the ordered bindings. An older binary cannot read the new bindings. Rollback requires restoring legacy preferences.

Blast Radius: keyboard

Assigned ordinary keys suppress their normal actions globally while enabled. Fn/Globe, Caps Lock, Shift, and multimedia keys cannot be assigned. Event-pairing regressions could affect typing or shortcuts.