feat(core+host+android): committed-text input plane (IME path) — InputKind::TextInput

The VK key-event vocabulary cannot express text an input method COMMITS
(autocorrect, gesture typing, non-Latin scripts, emoji). Add a first-class
text event and negotiate it:

- punktfunk-core: InputKind::TextInput (= 15) carries one Unicode scalar per
  event in `code`; HOST_CAP_TEXT_INPUT (0x04) in Welcome::host_caps.
- Host advertises the cap only where the session's inject backend can type
  text: Windows SendInput (KEYEVENTF_UNICODE, surrogate-pair aware) and the
  Linux wlroots backend — a dedicated second zwp_virtual_keyboard whose xkb
  keymap grows Unicode keysyms on demand (the wtype model), so keymap
  re-uploads never disturb the main device's layout/modifier state. The
  KWin-fake-input/libei/gamescope backends can only press layout keycodes, so
  those sessions don't set the bit and clients keep the VK fallback.
- GameStream plane: Moonlight's UTF-8 text packet (MAGIC_UTF8, previously
  recognized-and-dropped) now decodes to the same TextInput events.
- Android: KeyCaptureView picks a real editable InputConnection when the host
  has the cap — the IME runs its full machinery, mirrored to the host live via
  common-prefix diffs of the composition (backspaces + new suffix), with
  setComposingRegion adopting committed text so autocorrect-revert flows diff
  instead of retyping; newline→Enter, deleteSurroundingText→Backspace/Delete.
  Older hosts keep the TYPE_NULL raw-key path unchanged.
- keymap: media VKs (0xB0-0xB3) → evdev so the Android media keys land on
  Linux hosts too.

Verified: punktfunk-core + host gamestream + pf-inject tests green on Linux
(Ubuntu box), clippy clean; Android app+native builds.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-inject/src/inject/linux/libei.rs

@@ -438,6 +438,7 @@ fn kind_bit(kind: InputKind) -> u32 {
         InputKind::GamepadState => 12,
         InputKind::GamepadRemove => 13,
         InputKind::GamepadArrival => 14,
+        InputKind::TextInput => 15,
     };
     1 << i
 }
@@ -670,6 +671,9 @@ impl EiState {
             | InputKind::GamepadAxis
             | InputKind::GamepadRemove
             | InputKind::GamepadArrival => return, // uinput path (later)
+            // libei presses keycodes against the server's negotiated keymap — no committed-text
+            // path (the HOST_CAP_TEXT_INPUT cap is not advertised on this backend).
+            InputKind::TextInput => return,
         };
         self.injected += 1;
         let n = self.injected;
@@ -844,7 +848,8 @@ impl EiState {
             | InputKind::GamepadButton
             | InputKind::GamepadAxis
             | InputKind::GamepadRemove
-            | InputKind::GamepadArrival => emitted = false,
+            | InputKind::GamepadArrival
+            | InputKind::TextInput => emitted = false,
         }
 
         if emitted {