Fix DSD decoder respawn to match the audio engine's payload format

[ultraelectronica]

Summary

Fixes silent DSD playback after seeking, track changes, and requeues on DAPs that
run a native-DSD engine, and corrects WavPack PCM level normalization.

DSD respawns no longer infer their encoding from pipeline flags

• In-engine DSD decoders spawned for seek / next-track / requeue used to derive
  their output encoding from the pipeline mode. A native-DSD DAP engine runs its
  raw-bypass pipeline as PipelineMode::Dop, so a respawn inferred DoP and
  spawned a DoP decoder into a native stream. The native render loop reads each
  word's low byte, and a DoP word's low byte is always 0x00 — silence. The
  result was DSD audio that went quiet once you seeked or advanced.
• Record the payload format (Pcm / Dop / Native) at engine creation in a
  new EngineDsdMode on AudioCallbackData, fixed for the engine's lifetime
  because the format is burned into the stream. spawn_decoder now takes the
  engine's callback data and reads that recorded mode instead of re-deriving it
  from the current preference.
• Derive the initial mode from the requested output strategy: native and USB
  native DSD map to Native, DoP maps to Dop, and everything else (including
  the desktop/cpal engine) is Pcm.
• Force engine recreation when the caller excludes the current strategy (for
  example the DoP-to-PCM decimation fallback), even when the rate/strategy
  checks would otherwise reuse the handle — a mismatched payload format cannot be
  swapped in place.

WavPack PCM normalized by native bit depth

• libwavpack returns integer samples right-justified at the source's native bit
  depth (a 24-bit file yields ±2^23 even though it occupies three bytes), but the
  old scale keyed off the container width and normalized 3- and 4-byte samples by
  2^31. A 24-bit file was therefore scaled 256x too quiet.
• Add wavpack_pcm_scale(bits_per_sample, bytes_per_sample), which normalizes by
  the real bit depth (falling back to the container width when depth is missing,
  clamped to 1..=32). This also handles wasted-bit files such as 20-bit audio
  stored in a 24-bit container, and fixes the log line to report both values.

Tests

• engine_dsd_mode_roundtrips_and_defaults_to_pcm.
• dsd_respawn_matches_engine_payload_format_not_pipeline_flags, which writes a
  minimal stereo DSD64 DSF fixture and asserts a native engine respawns at
  352800 Hz (not the 176400 Hz DoP carrier the old inference chose), a DoP engine
  still respawns DoP, and a PCM-decimation engine respawns PCM.
• pcm_scale_normalizes_to_native_bit_depth covering 8/16/24/32-bit, 20-bit in
  a 24-bit container, and the missing-depth fallback.

Related issue

none

Checklist

• flutter analyze passes
• flutter test passes
• Rust changes: cargo test --manifest-path rust/Cargo.toml passes
• Built and ran the app if the change touches a playback path (flutter run --flavor full)
• Generated code regenerated and committed if bridge/drift/freezed models changed
• CHANGELOG.md updated for user-facing changes
• No secrets, keys, tokens, or machine-local files committed
• One change per PR; no unrelated reformatting in the same diff
• Hardware tested if the Rust audio engine or USB/DAC output changed