docs(plan): orienting note for the timestampOffset non-zero-PTS spike

Two-path context (native-PTS on feat/spf-non-zero-pts vs timestampOffset
relocation here), the four problems to keep in view, the load-behavior
coordinate-assumption audit, and TODOs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/plans/spf-non-zero-pts-timestamp-offset-spike.md

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+# Spike: non-zero-PTS via `timestampOffset` (relocate to 0-based)
+
+Exploratory. An alternative to the native-PTS approach spiked on
+`feat/spf-non-zero-pts`. Both paths stay live; this branch explores the other one.
+
+## Base
+
+Branched off `7b0d8bbdc` (`feat(spf): parse VTT X-TIMESTAMP-MAP`) — the
+mechanism-independent text foundation, needed either way. Deliberately *without*
+the native-PTS spike that sits on `feat/spf-non-zero-pts` (commits `7c0c89aeb`,
+`8be855fe9`): adapter B-translate, `seekable-window` / `track-seekable-window` /
+`seek-to-window-start`, `anchor-presentation-timeline` buffered-pts, parser
+`startTimeOffset`/`placeByOffset`, and the observed-native SourceBuffer actor.
+Those are all specific to "model native + translate at the adapter" and would
+carry conflicting assumptions here.
+
+## The two paths
+
+- **Native-PTS (other branch):** buffer holds native PTS; model re-origined onto
+  it; adapter translates the player-facing timeline to 0-based; engine models its
+  own `seekableWindow`.
+- **This branch — `timestampOffset` relocation:** set
+  `SourceBuffer.timestampOffset ≈ −origin` so native PTS 60 lands at 0. Buffer,
+  model, and `currentTime` are all 0-based → native `seekable [0,dur]` and
+  `duration = content` work without engine-side modeling. Relocate at region
+  boundaries (after `abort()` + a keyframe), never per-segment — see
+  `internal/decisions/mse-timestamp-offset.md` §3.
+
+## Problems the other branch surfaced — keep in view (likely changed, not gone)
+
+1. **Initial-load stall** (anchor-shift / origin-seek / flush mis-coordination) —
+   probably *sidestepped*: 0-based buffer + model + currentTime from the start
+   means no post-shift coordinate mismatch. Confirm it doesn't reappear elsewhere.
+2. **Seekable `[0,origin)` void + `duration = origin+content`** — both simplified
+   away by relocation. No adapter-modeled window, no duration correction.
+3. **AV/text coupling** — the one to get right. Relocating AV to 0-based (mechanism
+   A) means text cues need the X-TIMESTAMP-MAP delta applied to match
+   (`cue_final = cue_LOCAL + MPEGTS/90000 − origin`). The parser in the base feeds
+   this; wire it so text rebase tracks the AV offset.
+4. **Negative-DTS on Chromium** — the headline `timestampOffset` risk. `−origin`
+   could push the earliest B-frame DTS below 0 at the very start (hard append
+   failure on Chromium, not Firefox). Probe this first. Also: audio
+   sample-continuity across any discontinuity (decision doc §6).
+
+## Load behavior + implicit assumptions (flagged) — applies to BOTH paths
+
+The segment loader / `load-segments` behavior and the actor planning carry implicit
+coordinate assumptions (e.g. planning/flush relative to raw `currentTime`, the
+`getSegmentsToLoad`/`calculateForwardFlushPoint` window keyed off `currentTime` vs
+`track.segments`). On the native-PTS branch this produced the planning-floor half
+of the stall (still open there). Either direction may need to rework these
+assumptions — don't assume the loader is coordinate-neutral. Audit it explicitly
+as the spike progresses.
+
+## TODO
+
+- [ ] Stand up a sandbox harness (the other branch's `apps/sandbox/src/spf-non-zero-pts/`
+      isn't here) to drive the real provider on the Mux instant clip + Apple bipbop.
+- [ ] Probe negative-DTS on Chromium with `timestampOffset = −origin`.
+- [ ] Wire text-cue rebase (X-TIMESTAMP-MAP delta) to match the AV offset.
+- [ ] Audit the load behavior's coordinate assumptions.
+
+## Pointers
+
+- Other approach + full problem write-ups: branch `feat/spf-non-zero-pts`
+  (`8be855fe9`, `7c0c89aeb`).
+- Decision: `internal/decisions/mse-timestamp-offset.md` (§3 relocation mechanism;
+  §5 negative-DTS guard; §6 audio sample-continuity; Update(2026-06-26) steered
+  *away* from relocation for finite VOD — this spike revisits that).