debug(gamestream): surface the pointer plane's silent failure modes

An on-glass 'touch does nothing' currently leaves NO trace: log the first
SS_TOUCH per session at info (proves the client sends + we decode), and warn
when a packet carrying a pointer magic fails the body parse (layout mismatch)
instead of vanishing into the unknown-magic drop.

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

crates/punktfunk-host/src/gamestream/control.rs

@@ -289,6 +289,14 @@ fn on_receive(
             let _ = inj_tx.send(ev);
         });
         return;
+    } else if super::input::is_pointer_magic(&pt) {
+        // A pointer magic that failed the body parse — a layout mismatch against this
+        // client, exactly what an on-glass "touch/pen does nothing" needs surfaced.
+        tracing::warn!(
+            len = pt.len(),
+            "gamestream: SS_TOUCH/SS_PEN packet failed to decode (malformed/unexpected layout)"
+        );
+        return;
     }
 
     let events = super::input::decode(&pt);

crates/punktfunk-host/src/gamestream/input.rs

@@ -150,6 +150,18 @@ pub enum SsPointer {
     Touch(SsTouch),
 }
 
+/// Whether this control plaintext carries a pointer magic ([`decode_pointer`]'s domain) —
+/// lets the caller tell "malformed pointer packet" (worth a warn) apart from "some other
+/// message" when `decode_pointer` returns `None`.
+pub fn is_pointer_magic(plaintext: &[u8]) -> bool {
+    plaintext.len() >= 12
+        && u16::from_le_bytes([plaintext[0], plaintext[1]]) == INPUT_DATA_TYPE
+        && matches!(
+            u32::from_le_bytes([plaintext[8], plaintext[9], plaintext[10], plaintext[11]]),
+            MAGIC_SS_TOUCH | MAGIC_SS_PEN
+        )
+}
+
 /// Decode a control plaintext into a pen/touch pointer event, or `None` for every other
 /// message (the caller then falls through to [`decode`]). Bounds- and sanity-checked like the
 /// rest of the plane: short bodies and non-finite floats (a forged NaN must never reach the

crates/punktfunk-host/src/gamestream/pen.rs

@@ -67,6 +67,9 @@ pub struct GsPointer {
     saw_hover: bool,
     /// Active forwarded touch ids, for `CANCEL_ALL` replay as per-id ups.
     touch_ids: Vec<u32>,
+    /// Whether this session's first SS_TOUCH was logged (the one observable breadcrumb an
+    /// on-glass "touch does nothing" report needs — every later stage logs its own failure).
+    touch_seen: bool,
 }
 
 impl GsPointer {
@@ -80,6 +83,7 @@ impl GsPointer {
             last: PenSample::default(),
             saw_hover: false,
             touch_ids: Vec::new(),
+            touch_seen: false,
         }
     }
 
@@ -199,6 +203,13 @@ impl GsPointer {
     }
 
     fn apply_touch(&mut self, t: &SsTouch, mut sink: impl FnMut(InputEvent)) {
+        if !self.touch_seen {
+            self.touch_seen = true;
+            tracing::info!(
+                event_type = t.event_type,
+                "gamestream: touch plane active (first SS_TOUCH from this client)"
+            );
+        }
         let ev = |kind: InputKind, id: u32, x: f32, y: f32| InputEvent {
             kind,
             _pad: [0; 3],