[iterate-4A] intro-video: fix decode-timeout clock, feeder starvation, k_8 texture decode

Three layered root causes kept ADV.wmv from playing. This lands the first
three fixes: the intro video now decodes end-to-end and uploads its YUV
planes. The on-screen composite still needs a multi-texture render path
(root #3, tracked separately — the shader interpreter binds one texture slot
but the YUV->RGB pass samples three).

1. Clock scale (xenia-kernel/state.rs): INSTRUCTIONS_PER_MS 10_000 -> 1_000_000.
   KeTimeStampBundle tick_count = global_clock / INSTRUCTIONS_PER_MS. At 10_000
   (~10 MIPS; global_clock further inflated ~58x by the serialized scheduler
   summing busy-spin) the movie handler's 2000 ms software-decode deadline
   (sub_821B4968 @0x821b68c0) tripped on a legitimate ~20M-instruction
   720p-YUV420 decode and aborted playback (canary never enters that wait
   loop). 1_000_000 sits in the validated [~600k, ~2.77M] window that fits
   both the movie (decode <=2000 ms) and boot (the worker-hub +66 ms gate
   still elapses before the movie, ~66M instr). XENIA_INSTR_PER_MS overrides.

2. Scheduler fairness (xenia-cpu/scheduler.rs): pick_runnable's equal-priority
   tiebreak now prefers the incumbent (running_idx) so decrement_quantum's
   quantum rotation sticks. Previously each round re-picked the lowest index,
   so co-located equal-priority threads never alternated and the movie's demux
   feeder (tid24, co-located on hw=1) starved until the STARVE_LIMIT=4096
   backstop -- the decode ring never refilled. Priority preemption unaffected.
   XENIA_INCUMBENT_PICK=0 rolls back.

3. k_8 texture decode (xenia-gpu/texture_cache.rs + xenia-ui/texture_cache_host.rs):
   the video uploads its YUV420 planes as linear k_8 textures (Y 1280x720,
   U/V 640x360); with no k_8 decoder ensure_cached rejected them and the frames
   never reached the GPU. Adds decode_k8 (1 byte/texel expanded to Rgba8Unorm)
   + the host Rgba8Unorm mapping.

Read-only diagnostic probe knobs used to find these remain uncommitted in the
working tree. Boot goldens re-baselined in a follow-up commit (the clock change
intentionally moves the digest; see tests/golden/README.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xenia-ui/src/texture_cache_host.rs

@@ -162,6 +162,7 @@ impl TextureCacheHost {
 /// `bytes_per_row` computation branches on the block size too.
 fn descriptor_for(key: &TextureKey) -> Option<wgpu::TextureDescriptor<'static>> {
     let format = match key.format {
+        TextureFormat::K8 => wgpu::TextureFormat::Rgba8Unorm, // CPU-expanded (replicated)
         TextureFormat::K8888 => wgpu::TextureFormat::Rgba8Unorm,
         TextureFormat::K565 => wgpu::TextureFormat::Rgba8Unorm, // CPU-expanded
         TextureFormat::Dxt1 => wgpu::TextureFormat::Bc1RgbaUnorm,