feat(host+inject): pen P1 — per-session uinput virtual tablet, HOST_CAP_PEN live

The Linux injection leg of design/pen-tablet-input.md: 0xCC/0x05 pen batches now
route through the per-session PenTracker into a lazily-created 'Punktfunk Pen'
uinput tablet (BTN_TOOL_PEN/RUBBER, pressure, tilt-from-polar, ABS_Z barrel
roll, hover distance, INPUT_PROP_DIRECT) — compositors pick it up via libinput
and hand apps zwp_tablet_v2 with full fidelity. The host now advertises
HOST_CAP_PEN when /dev/uinput is accessible (PUNKTFUNK_PEN=0 kill-switch);
transitions group into SYN frames so proximity-enter carries its position.
Stroke failsafe: clients heartbeat ≤100ms while in range (documented wire
contract — capture APIs are silent for a stationary pen); 200ms of silence
force-releases. 'punktfunk-host pen-test' draws a pressure-ramped sine stroke
through the real tracker→uinput chain, no client needed.

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

include/punktfunk_core.h

@@ -1094,11 +1094,13 @@
 #endif
 
 #if defined(PUNKTFUNK_FEATURE_QUIC)
-// Host-side failsafe (design/pen-tablet-input.md §2): a tracker still touching after this many
-// ms without a sample force-releases ([`PenTracker::force_release`]) — a client that died
-// mid-stroke must not leave the host's virtual pen inked-down forever. Far above any real
-// send cadence (a touching pen streams samples continuously), so it never fires on a live
-// slow stroke.
+// Host-side failsafe (design/pen-tablet-input.md §2): a tracker still in range after this
+// many ms without a sample force-releases ([`PenTracker::force_release`]) — a client that
+// died mid-stroke must not leave the host's virtual pen inked-down forever. This makes the
+// **client heartbeat a wire contract**: capture APIs only fire on change, so a stationary
+// pen is naturally silent — senders MUST repeat the last sample at least every ~100 ms while
+// the pen is in range or touching (it re-decodes as pure Motion, harmless), keeping a live
+// stationary stroke two heartbeats clear of the deadline.
 #define PEN_TOUCH_TIMEOUT_MS 200
 #endif