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  - `vaapi::fourcc` — superseded by `pf_frame::drm_fourcc`; no call sites left.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/enc/windows/amf.rs

@@ -2760,6 +2760,10 @@ mod tests {
     }
 
     /// The `p`-quantile of `samples` (µs), sorting in place. `0` when empty.
+    /// Gated like its only caller, the `amf-qsv`-only §5.2 latency A/B below — otherwise a
+    /// `--features nvenc,qsv` build compiles this helper with the benchmark cfg'd out and trips
+    /// `dead_code` (which the crate root no longer blanket-allows).
+    #[cfg(feature = "amf-qsv")]
     fn percentile(samples: &mut [u128], p: f64) -> u128 {
         if samples.is_empty() {
             return 0;
@@ -2778,6 +2782,8 @@ mod tests {
     /// so its submit→AU is the bare ASIC time. The last ~2 unflushed frames on the ffmpeg path
     /// are left unmeasured (dropped with the encoder) so every recorded sample is a genuine paced
     /// submit→AU.
+    /// Gated like its only caller (see [`percentile`]).
+    #[cfg(feature = "amf-qsv")]
     #[allow(clippy::too_many_arguments)]
     fn drive_and_measure(
         enc: &mut dyn Encoder,