feat(host): in-place encoder rate reconfigure — ABR steps no longer cost an IDR

Every adaptive-bitrate step used to tear the encoder down and rebuild it,
opening on a full IDR (a 20-40x frame-size spike, in-flight AU forfeit and
an IDR-cooldown anchor) — exactly when the Automatic controller is climbing.
Encoder::reconfigure_bitrate(bps) retargets the LIVE encoder instead
(default false, so libavcodec/software paths keep the rebuild fallback,
which also still owns the bitrate clamping):

- Linux + Windows direct NVENC: nvEncReconfigureEncoder (added to the
  hand-rolled runtime EncodeApi tables) with resetEncoder=0 / forceIDR=0;
  the same init/config is re-authored via the new shared build_config/
  build_init_params with only avg/max bitrate + VBV (PUNKTFUNK_VBV_FRAMES)
  moved. On-hardware test: 20→60→10 Mbps in place, zero IDRs (RTX 5070 Ti).
- Native AMF: TargetBitrate/PeakBitrate/VBVBufferSize are dynamic
  properties — SetProperty on the live component, no Terminate/re-Init.
- Vulkan Video (HEVC + AV1): stage the rate and emit an
  ENCODE_RATE_CONTROL control command on the next recorded frame (begin
  keeps declaring the session's current state, as the spec requires).

The session glue tries the in-place retarget first and skips the rebuild/
inflight-clear/IDR-cooldown bookkeeping when it succeeds — the reference
chain and the wire-index prediction survive, so RFI keeps working across
rate steps.

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

crates/punktfunk-host/src/encode.rs

@@ -301,6 +301,17 @@ pub trait Encoder: Send {
     fn reset(&mut self) -> bool {
         false
     }
+    /// Retarget the encoder's rate control to `bps` (average == max, CBR) **in place** — same
+    /// codec/resolution/fps, only the bitrate and its derived VBV move. Returns `true` when the
+    /// live encoder accepted the change: the reference chain, the in-flight frames and the
+    /// caller's wire-index prediction all survive, so an adaptive-bitrate step costs *nothing* on
+    /// the wire (no IDR, no in-flight forfeit — the whole point vs. a rebuild). `false` = the
+    /// backend can't (or the driver rejected the new rate, e.g. above the codec-level ceiling) —
+    /// the caller falls back to its full rebuild path, which also owns the bitrate clamping.
+    /// Default: no in-place retarget (the libavcodec/software paths).
+    fn reconfigure_bitrate(&mut self, _bps: u64) -> bool {
+        false
+    }
     /// Signal end-of-stream. After this, drain the remaining AUs with [`poll`](Self::poll)
     /// until it returns `None` — NVENC buffers frames internally even at `delay=0`.
     fn flush(&mut self) -> Result<()>;