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/punktfunk-host/src/native/handshake.rs

@@ -444,6 +444,15 @@ pub(super) async fn negotiate(
                 punktfunk_core::quic::HOST_CAP_CLIPBOARD
             } else {
                 0
+            }
+            // Committed-text injection (InputKind::TextInput): only where the session's inject
+            // backend can actually type text — Windows SendInput (KEYEVENTF_UNICODE) and the
+            // Linux wlroots virtual keyboard (dynamic Unicode keymap). Clients without the bit
+            // keep their VK-synthesis fallback for IME text.
+            | if crate::inject::text_input_supported() {
+                punktfunk_core::quic::HOST_CAP_TEXT_INPUT
+            } else {
+                0
             },
         // The negotiated session AEAD (resolved above) + its 32-byte key toward a ChaCha
         // client; toward everyone else cipher 0 keeps the Welcome byte-identical to the