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/lib.rs

@@ -174,6 +174,29 @@ pub fn default_backend() -> Backend {
     Backend::Unsupported
 }
 
+/// Whether the session's inject backend can type **committed text**
+/// ([`InputKind::TextInput`] — see `HOST_CAP_TEXT_INPUT`): Windows always (`KEYEVENTF_UNICODE`);
+/// Linux only on the wlroots backend (a dedicated virtual keyboard with a dynamically-grown
+/// Unicode keymap) — KWin fake-input/libei/gamescope can only press keycodes of the host layout.
+/// Consulted at Welcome time to advertise the cap; a mid-session backend switch away from a
+/// capable one just degrades to dropped text events (input is lossy by design).
+#[cfg(target_os = "windows")]
+pub fn text_input_supported() -> bool {
+    true
+}
+
+/// See the Windows variant: Linux types text only through the wlroots virtual-keyboard backend.
+#[cfg(target_os = "linux")]
+pub fn text_input_supported() -> bool {
+    matches!(default_backend(), Backend::WlrVirtual)
+}
+
+/// No injector ⇒ no text.
+#[cfg(not(any(target_os = "linux", target_os = "windows")))]
+pub fn text_input_supported() -> bool {
+    false
+}
+
 #[path = "inject/service.rs"]
 mod service;
 pub use service::InjectorService;