docs(498): record QsvPreferNativeDecoder decision

Field-reference update skipped: docs/channels.md and docs/domain-model.md
have no per-field FFmpeg-profile catalogue (channels.md's Encoding bullet
is a one-line summary, not a field list; grep for QsvExtraHardwareFrames
or HardwareAcceleration finds no such list in either doc).

docs/decisions.md

@@ -2401,3 +2401,30 @@ shared row already delivers the user-visible behavior with no schema change.
 
 **Accepted trade-off:** every channel on the shared preset shares one geometry; per-channel tweaks mean
 creating a second preset on the Watermarks screen.
+## 2026-07-20 (#498) — QSV decode is split from QSV encode via a single `QsvPreferNativeDecoder` bool
+
+`FFmpegProfile.HardwareAcceleration` picked one pipeline builder for **both** decode and encode, so an
+Intel QSV profile decoded with the QSV decoder — which is materially less tolerant of imperfect H.264 than
+FFmpeg's native VA-API decoder and cannot carry Dolby Vision metadata. Jellyfin, on identical hardware,
+avoids this by decoding with VA-API and encoding with QSV; that combination was not expressible in
+ErsatzTV. Full design: `docs/superpowers/specs/2026-07-20-qsv-native-decode-design.md`.
+
+- **Chosen: a single, QSV-scoped `FFmpegProfile.QsvPreferNativeDecoder` boolean**, mirroring Jellyfin's
+  "Prefer OS native DXVA or VA-API hardware decoders" checkbox exactly — Jellyfin itself deliberately
+  collapses this to one default-on toggle rather than a decode-family picker, and copying the reference
+  tool's granularity avoids over-building past a problem it has already solved.
+- **Default ON.** Native VA-API decode is strictly more tolerant of imperfect streams than the QSV decoder
+  wrapper, and Dolby Vision passthrough requires it — so the working configuration *is* the default for a
+  reliability fix. Existing QSV profiles adopt the hybrid on upgrade via the migration's
+  `HasDefaultValue(true)` (SQLite `INTEGER`/MySQL `tinyint` `NOT NULL DEFAULT 1`), no opt-in required.
+- **Rejected: a general `DecodeHardwareAcceleration` enum column** decoupling decode and encode hardware
+  families entirely. More configurable than Jellyfin's own control, but nothing in the codebase or the
+  reported failure needs that generality yet — `FFmpegState` already separates
+  `DecoderHardwareAccelerationMode` / `EncoderHardwareAccelerationMode` internally (used today for the
+  software-decode-plus-hardware-encode error-loop fallback), so the seam to grow into a real enum exists
+  if a second asymmetric-decode case ever shows up on another hardware family. Deferred rather than
+  built now, on YAGNI grounds.
+- **Contained to the QSV builder.** The flag does not touch `PipelineBuilderFactory`'s single-builder
+  dispatch per profile, and does not repurpose the existing decode/encode-mode fields into a general
+  cross-family selector — that repurposing is exactly the rejected enum option, just introduced through
+  the back door.