feat(android): pen P5 — stylus capture onto the pen plane

The Android leg of design/pen-tablet-input.md §7: against a HOST_CAP_PEN host,
stylus/eraser pointers split out of BOTH touch models (passthrough + gesture)
into StylusStream — state-full samples with pressure, AXIS_TILT, azimuth from
AXIS_ORIENTATION (Android's 0 = away-from-user IS wire north), AXIS_DISTANCE
hover, both stylus barrel buttons, the eraser tool, and historical (coalesced)
samples batched oldest-first. Kotlin heartbeats ≤100ms per the wire contract.
JNI: nativeHostSupportsPen + nativeSendPen (flat 10-float stride, sentinels
<0). No barrel-roll axis exists on Android — roll stays unknown here.

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

clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt

@@ -287,6 +287,22 @@ object NativeBridge {
     /** One key transition. vk: Windows VK (0 = dropped by Rust). mods: VK modifier mask (0 for now). */
     external fun nativeSendKey(handle: Long, vk: Int, down: Boolean, mods: Int)
 
+    /**
+     * Whether the host advertised full-fidelity stylus injection (`HOST_CAP_PEN`) — the gate
+     * for splitting stylus pointers out of the touch path onto the pen plane. False on `0`.
+     */
+    external fun nativeHostSupportsPen(handle: Long): Boolean
+
+    /**
+     * One stylus batch of STATE-FULL samples (the pen plane; design/pen-tablet-input.md §7):
+     * [count] × 10 floats, oldest first — `[state, tool, x, y, pressure, distance, tilt_deg,
+     * azimuth_deg, roll_deg, dt_us]`. `state` = the wire in-range/touching/barrel bits; `tool`
+     * 0=pen 1=eraser; x/y/pressure/distance normalized 0..1; distance/tilt/azimuth/roll < 0 =
+     * unknown. Send only when [nativeHostSupportsPen]; repeat the last sample ≤100 ms while the
+     * pen is in range (the host force-releases a silent stroke after 200 ms).
+     */
+    external fun nativeSendPen(handle: Long, samples: FloatArray, count: Int)
+
     /**
      * Whether the host advertised committed-text injection (`HOST_CAP_TEXT_INPUT`) — its inject
      * backend can type Unicode text directly. Picks the real IME `InputConnection` (autocorrect,