diag(vkdecode): log the driver's video capabilities verbatim

Nothing in the caps module logged anything, so when a device refused with
"advertises neither DPB_AND_OUTPUT_COINCIDE nor DISTINCT" there was no way to
separate two very different situations that present identically as a zero: the
driver filling the chain and genuinely declaring no DPB mode, versus our own
pNext chain never reaching VkVideoDecodeCapabilitiesKHR at all.

Printing the BASE VkVideoCapabilitiesKHR beside the decode flags is the
discriminator. A populated max_dpb_slots next to decode_flags: 0 means the
driver traversed the chain and answered; zeros across both mean the query never
landed and the refusal is ours, not the driver's.

Raised by the Intel Arc result on .221, where I concluded "driver bug" on the
strength of our own code's report — which is precisely the circular reasoning
this line exists to break.

crates/pf-vkdecode/src/caps_h265.rs

@@ -308,6 +308,26 @@ pub(crate) unsafe fn query_h265_caps(
     let decode_flags = decode_caps.flags;
     let max_level_idc = h265_caps.max_level_idc;
 
+    // What the driver ACTUALLY said, before any of our interpretation. Nothing in this
+    // module logged, so a refusal downstream ("advertises neither COINCIDE nor DISTINCT")
+    // was indistinguishable from our own chain never reaching the struct: both present as
+    // a zero. Printing the BASE capabilities beside the decode ones is the discriminator —
+    // a populated `max_dpb_slots` next to `decode_flags: 0` means the driver filled the
+    // chain and genuinely declared no DPB mode; zeros across both mean the query never
+    // landed. Debug rather than info: one line per profile per session, wanted only when
+    // someone is asking this exact question.
+    tracing::debug!(
+        codec = "H.265",
+        ?capability_flags,
+        ?decode_flags,
+        max_dpb_slots,
+        max_active_reference_pictures,
+        ?min_coded_extent,
+        ?max_coded_extent,
+        ?picture_access_granularity,
+        "driver video capabilities, verbatim"
+    );
+
     // The three queries carry the REAL creation usages (SAMPLED included for the
     // presenter-facing roles) so the answers validate the images the pools build.
     let decode_profile = DecodeProfile::H265(key);