docs(video): three-backend reality + log software-by-preference

The silent settings-driven software path cost a debugging round on the
first Vulkan Video glass test (stale decoder=software from the VAAPI-
broken-on-NVIDIA era) — now it says so. Module docs updated: auto is
vulkan → vaapi → software.

Validated on glass (RTX 5070 Ti, HEVC 4K@144): decode 11 ms → 0.1 ms,
e2e p50 ~115 ms → 8.6 ms (the old number was software-decode queueing,
not clock skew), 144 fps locked, 2686 frames, zero errors.

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

crates/pf-client-core/src/trust.rs

@@ -252,7 +252,8 @@ pub struct Settings {
     /// preference — the host honors it when it can emit it, else falls back to the best shared codec.
     #[serde(default = "default_codec")]
     pub codec: String,
-    /// Video decoder preference: `"auto"` (VAAPI → software), `"vaapi"`, `"software"`.
+    /// Video decoder preference: `"auto"` (Vulkan Video → VAAPI → software),
+    /// `"vulkan"`, `"vaapi"`, `"software"`.
     /// The `PUNKTFUNK_DECODER` env var overrides this (see `video::Decoder::new`).
     pub decoder: String,
     /// Show the on-stream statistics overlay (toggle live with Ctrl+Alt+Shift+S).