chore(kwin-composite): TEMP blend-path probes — DROP BEFORE MERGE

Rate-limited journal logging bracketing the silent segment of the Linux
composite path: what overlay (if any) the encode loop's composite arm
hands the encoder, and whether the NVENC cursor-blend kernel launches and
with what geometry. On-glass (KWin leg) the blend module loads yet the
composite cursor stays invisible — these two probes attribute it to
no-overlay / stripped-en-route / kernel-draws-nothing. The module-loaded
INFO line is permanent (success must be as attributable as failure);
everything else in this commit is temporary.

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

crates/punktfunk-host/src/native/stream.rs

@@ -2077,6 +2077,34 @@ pub(super) fn virtual_stream(ctx: SessionContext, prepared: Option<PreparedDispl
                 if let Some(live) = capturer.cursor() {
                     frame.cursor = Some(live);
                 }
+                // TEMP KWin composite probe (DROP BEFORE MERGE): what the composite arm actually
+                // hands the encoder — on-glass the blend module loads yet nothing draws, so the
+                // open question is whether an overlay reaches the frame at all.
+                #[cfg(not(target_os = "windows"))]
+                {
+                    use std::sync::atomic::{AtomicU64, Ordering as ProbeOrd};
+                    static PROBE_COMPOSITE: AtomicU64 = AtomicU64::new(0);
+                    let n = PROBE_COMPOSITE.fetch_add(1, ProbeOrd::Relaxed) + 1;
+                    if n == 1 || n % 512 == 0 {
+                        match frame.cursor.as_ref() {
+                            Some(c) => tracing::info!(
+                                n,
+                                x = c.x,
+                                y = c.y,
+                                w = c.w,
+                                h = c.h,
+                                visible = c.visible,
+                                serial = c.serial,
+                                "composite probe: overlay riding to the encoder"
+                            ),
+                            None => tracing::info!(
+                                n,
+                                "composite probe: NO overlay on the frame (capturer.cursor() None \
+                                 and no frame-attached overlay)"
+                            ),
+                        }
+                    }
+                }
             }
         }
         // The overlay surfaces hidden pointers too (for the hint above) — strip them