feat(encode): RGB-direct (EFC) is now the DEFAULT on capable hosts — gated by a session cursor-blend hint

B2 default-on (design/vulkan-rgb-direct-encode.md): wherever the probe
passes, Vulkan Video sessions now take the EFC RGB source by default —
direct import on aligned modes, padded-copy staging on unaligned ones
(1080p). PUNKTFUNK_VULKAN_RGB_DIRECT becomes the override: =0 disables,
=1 forces, unset = the default below.

The one session class that must NOT default on: cursor-as-metadata
captures (every non-gamescope compositor), where the CSC shader's blend
IS the visible pointer — the EFC cannot composite, and defaulting there
would silently drop the cursor from the stream. The hint rides the
existing plumbing:

- SessionPlan gains cursor_blend, resolved once where the compositor is
  known (gamescope embeds the pointer itself → false; kwin/mutter/
  wlroots/hyprland → true), and shows up in the logged plan line.
- open_video/open_video_backend thread it through (native pump: all
  three encoder-open sites read plan.cursor_blend; GameStream monitor
  capture: true — it negotiates metadata cursor; spike: false).
- VulkanVideoEncoder::open resolves: env override, else ON iff the
  session never hands us cursor bitmaps. The warn-once for a cursor on
  an RGB session (forced via =1) stays.

Verified on-hw box (Linux): pf-encode + punktfunk-host compile, clippy
clean, unit suite green. The GPU paths themselves are unchanged from the
smoke-validated 96e19986 — this commit only changes which sessions
select them by default.

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

crates/punktfunk-host/src/session_plan.rs

@@ -103,6 +103,11 @@ pub struct SessionPlan {
     /// PyroWave session — applied to EVERY encoder this plan opens (initial + all rebuilds) so
     /// AUs stay shard-aligned across mode/bitrate/stall rebuilds. `None` for the H.26x codecs.
     pub wire_chunk: Option<usize>,
+    /// The session may hand the encoder cursor bitmaps to composite (cursor-as-metadata
+    /// captures — every non-gamescope compositor; gamescope embeds the pointer itself).
+    /// Encoders whose fast path cannot blend (the Vulkan EFC RGB-direct source) stay on their
+    /// blending path when this is set, so the pointer never silently vanishes from the stream.
+    pub cursor_blend: bool,
 }
 
 impl SessionPlan {
@@ -112,6 +117,7 @@ impl SessionPlan {
         bit_depth: u8,
         chroma: crate::encode::ChromaFormat,
         codec: crate::encode::Codec,
+        cursor_blend: bool,
     ) -> Self {
         SessionPlan {
             capture: CaptureBackend::resolve(),
@@ -122,6 +128,7 @@ impl SessionPlan {
             chroma,
             codec,
             wire_chunk: None,
+            cursor_blend,
         }
     }