refactor(encode): drop the crate-wide allow(dead_code)

Inherited from the pre-extraction host crate root as scaffolding for backend
paths defined ahead of the build that used them. A census across every feature
combination on both platforms (flip it to `warn`, rebuild) found it was hiding
exactly two items — so it bought nothing while blinding the crate to future rot:

  - `vaapi::fourcc` — superseded by `pf_frame::drm_fourcc`; no call sites left.
  - `vulkan_video::open_opts` — test-only (the smoke tests use it to pass the
    RGB-direct request explicitly rather than through the env), now `#[cfg(test)]`.

Removing it surfaced a real latent defect the Linux census could not see:
`amf.rs`'s `percentile` / `drive_and_measure` helpers are used only by the
`#[cfg(feature = "amf-qsv")]` latency A/B benchmark, so a `--features nvenc,qsv`
build compiled the helpers with their caller gated out. They now carry the same
gate as their caller.

Verified `--all-targets -D warnings` on Linux (no features; shipped
nvenc+vulkan-encode+pyrowave) and Windows (no features; pyrowave; qsv; nvenc,qsv).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/pf-encode/src/lib.rs

@@ -5,9 +5,11 @@
 //! VA surface). One [`Encoder`] trait, selected in [`open_video`]. Extracted into a subsystem crate
 //! (plan §W6): depends on the shared frame vocabulary (`pf-frame`) + zero-copy plumbing
 //! (`pf-zerocopy`), never on capture — the capture→encode edge is one-way.
-// Scaffold: some backend paths + trait defaults are defined ahead of the per-feature build that
-// uses them (mirrors the host crate root's allow before the extraction).
-#![allow(dead_code)]
+// NOTE: no crate-wide `#![allow(dead_code)]`. It was inherited from the pre-extraction host crate
+// root as scaffolding for backend paths defined ahead of the build that used them, but a census
+// across every feature combination on both platforms found it was hiding exactly two items — so it
+// bought nothing and blinded the crate to future rot. Genuinely test-only helpers carry
+// `#[cfg(test)]` instead.
 // Every unsafe block in this module tree carries a `// SAFETY:` proof; enforce it (unsafe-proof
 // program). As a parent module this also covers the child modules (windows/linux backends).
 #![deny(clippy::undocumented_unsafe_blocks)]