docs: record Phase 18 (Opus low-data streaming) in COMPLETED; stage PLAN for Phase 21

COMPLETED.md

@@ -6,6 +6,32 @@ Newest entries at the top. Group by phase/wave header (mirroring `PLAN.md` / `CM
 
 ---
 
+## Phase 18 — Opus Low-Data Streaming (landed 2026-06-23)
+
+_Note: Two distinct efforts share the "Phase 18" label — phase numbers are organisational, not sequential. See also **Phase 18 — Theme / Dark-Mode Remediation (landed 2026-06-19)** below._
+
+**Landed:** 2026-06-23 on `streaming-overhaul`.
+
+- **What:** Dual-format audio delivery — the existing lossless WAV path plus a new low-data Ogg Opus (fullband, 320 kbps) path — giving listeners a quality choice. Opus is the default (capability-gated). Six waves, all landed.
+
+- **Why:** Lossless WAV works but imposes high bandwidth cost, particularly for long DJ Mixes. Opus at 320 kbps is indistinguishable for typical listening and an order of magnitude smaller, making the site usable on metered or constrained connections. The bespoke Web Audio decode→schedule graph is retained by deliberate choice; Opus feeds the same `IFormatDecoder` seam rather than an HTML `<media>` element or MSE.
+
+- **Shape (by wave):**
+  - **18.1 — Ingest transcode + seek-index sidecar.** New `OpusTranscodeService`/processor in `DeepDrftContent`, invoked post-store from `UnifiedTrackService.UploadAsync` as a background job (failure-tolerant; the track plays lossless first). Produces the Ogg Opus 320 artifact and a combined seek/setup **sidecar** (0.5 s-bucketed granule→byte index + `OpusHead`/`OpusTags` setup header), both stored in a new `track-opus` vault. **ffmpeg/libopus** added to the deploy bootstrap as a runtime prereq on the API host.
+  - **18.2 — Storage + lookup contract.** Server-side "given `EntryKey` + format, return the right artifact + content-type, with lossless fallback when no Opus artifact exists."
+  - **18.3 — Delivery: `?format=opus|lossless` + sidecar serving + proxy threading.** `?format=` param on the stream endpoint (Range preserved); sidecar-serving endpoint; param forwarded through `TrackProxyController`; `TrackMediaClient` sends the format param.
+  - **18.4 — Client Opus decoder + index-based seek + capability detection.** New `OpusFormatDecoder` (`IFormatDecoder`): Ogg-page demux, setup-header carry from the cached sidecar, `calculateByteOffset` binary-searching the precomputed seek index (VBR-safe accurate seek — not interpolation). Capability gate falls back to the lossless path on browsers without Opus support. Testing-phase fixes also landed: a valid ffmpeg-generated capability-probe sample and correct duration reporting (fixed the seekbar + visualizer in Opus mode).
+  - **18.5 — End-to-end integration.** Player format selection (Opus-by-default, capability-gated); sidecar fetch/inject on track load; **Backfill-Opus** CMS bulk action (third sibling to Generate-Profiles / Backfill-High-res); replace-audio regenerates Opus + sidecar.
+  - **18.6 — Settings menu + quality toggle + CMS status.** New public-site **Settings menu** (app-bar trigger + settings-item abstraction + `ListenerSettings`, persisted via the dark-mode-pattern seam: `streamQuality` cookie → prerender-read → `PersistentComponentState` → client cookie service); streaming-quality toggle (Low-data/Lossless) as its first occupant. CMS **Opus-status badge** on the track grid. CMS upload meter **Post-Processing** phase visualizing background Opus transcode.
+
+- **As-built divergence — the Opus decoder (important).** The originally-spec'd "segment-and-decode-each-page via `decodeAudioData`" model was implemented and then **replaced**: browser testing showed it glitches at every segment boundary and broke seeking (Opus has pre-skip + inter-frame state, so independent per-segment decode is architecturally wrong). The landed decoder is a **WebCodecs `AudioDecoder` streaming pipeline** (Ogg demux → packets → stateful continuous decode), with index-driven seek and a WebCodecs-based capability gate. Daniel's call at the decision point — offered a decode-whole stopgap (Option A) vs. the proper WebCodecs streaming decoder (Option B), he chose B: "Let's go for B, why do you like half-solutions so much" (the same no-stopgap posture as the Mix visualizer WebGL2 decision, where he said "no stopgap — WebGL as step 1, no pussyfooting"). The segmented approach is recorded as tried-and-replaced.
+
+- **What Phase 18 does NOT include:** windowed / bounded-memory streaming. Phase 18 still fetches the whole (smaller) Opus artifact; bounding client memory to a sliding forward window is **Phase 21's job**.
+
+- **Design memo:** `product-notes/phase-18-opus-low-data-streaming.md`.
+
+---
+
 ## Phase 20 — Theater Mode (landed 2026-06-20)
 
 **Landed:** 2026-06-20 on dev. Pending: final manual browser/GPU smoke-test on dev.
@@ -35,6 +61,8 @@ Newest entries at the top. Group by phase/wave header (mirroring `PLAN.md` / `CM
 
 ## Phase 18 — Theme / Dark-Mode Remediation (landed 2026-06-19)
 
+_Note: Two distinct efforts share the "Phase 18" label — phase numbers are organisational, not sequential. See also **Phase 18 — Opus Low-Data Streaming (landed 2026-06-23)** above._
+
 **Landed:** 2026-06-19 on dev (Wave 1 + Wave 2 + Wave 3).
 
 - **What:** A DRY token pass resolving six theming symptoms (five in dark mode, one in light) that all traced to three root causes: neutral page surfaces bound to constant brand tokens, the play chip bound to a constant light-grey, and no theme-aware popover-surface token. Resolved as one coherent pass via a shared token layer rather than per-component patches.