docs: move iframe player and backward seek to COMPLETED.md

COMPLETED.md

@@ -6,6 +6,35 @@ Newest entries at the top. Group by phase/wave header (mirroring `PLAN.md` / `CM
 
 ---
 
+## Embeddable iframe player
+
+**Status:** Feature complete on 2026-06-07 (commit `c83b132 feature: Embed Frame Player`, merged to dev).
+
+A standalone, chrome-free player surface intended for embedding in an `<iframe>` on external pages (e.g. a Bandcamp-style "play this track here" widget on a third-party blog or the collective's socials). Distinct from the dock player, which lives inside the full site chrome.
+
+**Shape as implemented:**
+
+- `Layout/EmbedLayout.razor` — a minimal layout: `MudThemeProvider` + `AudioPlayerProvider` wrapping `@Body`, with no nav, menu, or marketing chrome. Reuses the dark-mode `PersistentComponentState` round-trip (`CONTEXT.md §3.6`) so an embedded player still honours the theme.
+- `Pages/FramePlayer.razor` — routed at `/FramePlayer`, uses `EmbedLayout`, renders a single `<AudioPlayerBar Fixed />`. Reads a `TrackEntryKey` from the query string and auto-selects that track on load.
+- `Services/ITrackDataService.cs` + `TrackClientDataService.cs` — a new track-metadata fetch seam (`GetPage` + `GetTrack(trackId)`) so a component can resolve a single track by key without the gallery VM. Render-mode-agnostic (one seam, SSR and WASM both served by it).
+
+**Why it matters:** An embeddable player turns every external mention of a DeepDrft track into a play surface. It is the lightest-weight distribution lever the product has — no app install, no account, just a link that plays. Fits the collective's "get the music in front of people" posture.
+
+**Deferred:** CORS for arbitrary external embedders — handle when a concrete external host requires it.
+
+---
+
+## Phase 1.1 — Backward seek
+
+**Status:** Landed on 2026-06-07 (commits `daa334a`, `8581103` on seek-fix branch, merged to dev).
+
+- **What:** Seeking to a position *below* `playbackOffset` currently clamps silently to the start of the in-memory buffer segment instead of going to the user's chosen time. The forward "seek beyond buffer" path already exists in `WavOffsetService` + the client's offset-request path; backward seek is the missing mirror.
+- **Why it matters:** The single highest-impact missing feature in the player. Scrub-bar drags backward feel broken — they appear to seek but land in the wrong place.
+- **Shape:** Reuse the existing `GET api/track/{id}?offset=` pathway. The client decision becomes "is the target inside the decoded window?" — if yes, jump within the buffer (existing behaviour); if no (forward or backward), tear down the decoder and re-request from the byte-aligned offset.
+- **Implementation:** `WaveformSeeker` control supports both forward and backward seeking. The seek logic decides whether to jump within the decoded buffer or tear down and re-request from a byte-aligned offset regardless of direction. Backward seek observes the same `blockAlign` rounding-down as forward seek (enforced in `WavOffsetService.alignedOffset` and `StreamDecoder.calculateByteOffset`). Teardown/reinit respects the generation-counter pattern introduced by the concurrent-seek fix.
+
+---
+
 ## Phase 6 — Responsive home page (mobile layout)
 
 **Status:** All six slices landed on 2026-06-07 (branches `home-mobile-grid`, `home-mobile-hero`, `home-mobile-cta`, merged to dev).