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>

include/punktfunk_core.h

@@ -498,6 +498,18 @@
 #define HOST_CAP_CLIPBOARD 2
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// [`Welcome::host_caps`] bit: the host's active inject backend can type **committed text**
+// ([`InputKind::TextInput`](crate::input::InputKind::TextInput) — one Unicode scalar per event):
+// Windows (`KEYEVENTF_UNICODE`) and Linux wlroots (dynamic Unicode keymap on a dedicated virtual
+// keyboard); the KWin/libei/gamescope backends can only press layout keycodes, so those sessions
+// don't set it. A capable client routes its IME's committed text (autocorrect, gesture typing,
+// non-Latin scripts, emoji) through `TextInput` instead of lossy VK synthesis; absent the bit it
+// keeps the VK fallback. Packs into the existing trailing `host_caps` byte — no wire-layout
+// change; an older host ignores the unknown input tag anyway (input is lossy by design).
+#define HOST_CAP_TEXT_INPUT 4
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // [`Hello::video_codecs`] bit: the client can decode H.264 / AVC. The GPU-less **software**
 // encode path (openh264) emits H.264, so a client that wants to stream from a software host MUST
@@ -1094,6 +1106,17 @@ enum PunktfunkInputKind
     // [`HOST_CAP_GAMEPAD_STATE`](crate::quic::HOST_CAP_GAMEPAD_STATE); an older host ignores the
     // unknown tag (every pad then uses the session-default kind — the pre-existing behaviour).
     PUNKTFUNK_INPUT_KIND_GAMEPAD_ARRIVAL = 14,
+    // One Unicode scalar of **committed text** — `code` = the scalar value, everything else 0.
+    //
+    // The IME path: the layout-independent VK key events cannot express text an input method
+    // *commits* (autocorrect, gesture typing, non-Latin scripts, emoji), so a capable client
+    // sends the committed characters verbatim and the host injects them directly (Windows
+    // `KEYEVENTF_UNICODE`; Linux wlroots via a dynamically-grown Unicode keymap on a dedicated
+    // virtual keyboard). A multi-character commit is consecutive events in order. Sent only when
+    // the host advertised [`HOST_CAP_TEXT_INPUT`](crate::quic::HOST_CAP_TEXT_INPUT) — toward an
+    // older host (or one whose inject backend can't type text) clients keep the best-effort VK
+    // synthesis, and an older host ignores the unknown tag entirely.
+    PUNKTFUNK_INPUT_KIND_TEXT_INPUT = 15,
 };
 #ifndef __cplusplus
 #if __STDC_VERSION__ >= 202311L