fix(encode): RGB-direct must not engage on unaligned modes — the EFC reads past the capture buffer (GPU hang)

Field report (commit 25624443, RDNA2/VCN3 host, 1080p): repeated VCN0 VM
protection faults from punktfunk-host, vcn_enc_0.0 ring timeouts, two ring
resets, then a failed reset escalating to a full MODE1 GPU reset with VRAM
loss. Root cause is B1's RGB-direct source: the session's coded extent is
64x16-aligned (1920x1088) but the captured dmabuf is only the real mode
size (1920x1080) — the VCN EFC reads the 8 padding rows past the end of
the buffer (8 x 7680 B = 61 KB, matching the faulting page span exactly).
The CSC shader absorbs alignment by clamping reads and duplicating edges;
RGB-direct has no such stage. Every prior validation surface happened to
be aligned (256x256 smoke, 1440p live box) — 1080p is the common mode
that is not. The stall watchdog's rebuild-and-refault loop then turned a
per-session fault into the GPU death spiral.

Two layers:
- Open-time gate: RGB-direct engages only when the real mode equals the
  aligned coded extent (720p/1440p/4K yes, 1080p no); otherwise the CSC
  path runs and the verdict line says why (unaligned-mode). The padded-
  copy variant that re-enables 1080p is design-doc B2 work.
- Submit-time check: an RGB-direct frame that does not cover the coded
  extent errors out (encoder-rebuild path) instead of importing an
  undersized source.

On-glass (780M, host Mesa 26.0.4): all four smokes pass aligned; at
PF_SMOKE_W/H=250 the gate refuses RGB-direct and the rgb smokes soft-skip.
clippy clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/pf-encode/src/enc/linux/vulkan_video.rs

@@ -526,18 +526,33 @@ impl VulkanVideoEncoder {
         // the session's picture format. The probe runs unconditionally: its verdict is the
         // field telemetry that decides where B2 can default this on.
         let rgb_probe = probe_rgb_direct(&instance, &vq_inst, pd, codec_op, av1);
-        let rgb_cfg: Option<RgbDirect> = match (&rgb_probe, want_rgb) {
-            (Ok((x, y)), true) => Some(RgbDirect {
+        // ALIGNMENT GATE (field GPU-hang, 2026-07-20): the coded extent is 64x16-aligned but the
+        // captured dmabuf is only the REAL mode size — handing it to the encoder as the direct
+        // source makes the VCN's EFC read the alignment-padding rows PAST the end of the buffer.
+        // At 1920x1080 (coded 1088) that is 8 rows = 61 KB of out-of-bounds reads per frame:
+        // deterministic VM protection faults, vcn_enc ring timeouts, and — through the stall
+        // watchdog's rebuild-and-refault loop — a full MODE1 GPU reset with VRAM loss. The CSC
+        // shader absorbs the padding by clamping reads and duplicating the edge; RGB-direct has
+        // no such stage, so until the padded-copy variant lands (design doc B2) it only engages
+        // when the mode is already aligned (720p/1440p/4K are; 1080p is NOT).
+        let aligned = rw == w && rh == h;
+        let rgb_cfg: Option<RgbDirect> = match (&rgb_probe, want_rgb, aligned) {
+            (Ok((x, y)), true, true) => Some(RgbDirect {
                 x_offset: *x,
                 y_offset: *y,
             }),
             _ => None,
         };
         tracing::info!(
-            rgb_direct = match (&rgb_probe, &rgb_cfg) {
-                (_, Some(_)) => "active",
-                (Ok(_), None) => "available(off; set PUNKTFUNK_VULKAN_RGB_DIRECT=1)",
-                (Err(e), None) => e,
+            rgb_direct = match (&rgb_probe, want_rgb, aligned, &rgb_cfg) {
+                (_, _, _, Some(_)) => "active",
+                (Ok(_), true, false, None) =>
+                    "unaligned-mode(coded extent is 64x16-aligned; direct source would read \
+                     past the capture buffer — CSC path used; padded-copy variant is B2)",
+                (Ok(_), false, _, None) => "available(off; set PUNKTFUNK_VULKAN_RGB_DIRECT=1)",
+                (Err(e), _, _, None) => e,
+                // (Ok, wanted, aligned) always builds Some above.
+                (Ok(_), true, true, None) => unreachable!("rgb gate and cfg disagree"),
             },
             "vulkan-encode: EFC RGB-direct encode source (design/vulkan-rgb-direct-encode.md)"
         );
@@ -1753,6 +1768,21 @@ impl VulkanVideoEncoder {
         }
         let (src_img, src_view, acquire, compute_active) = match &frame.payload {
             FramePayload::Dmabuf(d) => {
+                // Defense in depth for the OOB class the alignment gate closes at open: the
+                // imported buffer must cover the FULL coded extent, or the EFC reads past it
+                // (VM faults → VCN ring hang → GPU reset, the 2026-07-20 field report). A
+                // mismatched frame (mid-flight mode change, odd capture) errors out here and
+                // takes the encoder-rebuild path instead of faulting the GPU.
+                if frame.width != self.width || frame.height != self.height {
+                    bail!(
+                        "vulkan-encode (rgb-direct): frame {}x{} does not cover the coded \
+                         extent {}x{} — refusing an out-of-bounds encode source",
+                        frame.width,
+                        frame.height,
+                        self.width,
+                        self.height
+                    );
+                }
                 let (img, view, fresh) = self.import_cached(d, frame.width, frame.height)?;
                 let acq = if fresh {
                     SrcAcquire::RgbFresh