Feature: Hold-to-modifier

**Summary**

New action type: press a bound button → modifier key-down (Super/Mod held); release the button → modifier key-up. Currently impossible — KeyCombo requires a non-modifier key, and press_key fires down+up instantly.

**Use case**

Hold the MX Master 4 haptic panel → Super is held → scroll wheel switches workspaces (GNOME/KDE). Release → Super released. No timer — modifier follows the physical button.

**Blockers**

1. KeyCombo parser rejects modifier-only chords (MissingKey).
2. Inject backends emit down+up back-to-back — no split hold/release path.

**Proposed**

Action::HoldModifier(u8) — modifier bitmask reusing existing MOD_* flags. Capture layer already sees press/release; dispatch calls hold_modifier_down(bits) on press, hold_modifier_up(bits) on release. All three backends have the primitives.

Trace the KeyCombo parser, press_key path, capture layer, dispatch logic, and the three inject backends mentioned in the issue. Confirm how modifier bits and button press/release events flow before implementing the action. Done means modifier-only bindings parse successfully, key-down occurs on button press, key-up on release, and all three backends preserve the hold rather than emitting an immediate pair.

Written by the indexing model from the issue text.