feat(slideshow): per-slide autoplay (manual-advance, opt-in) (#1708)

* feat(slideshow): per-slide autoplay (manual-advance, opt-in)

Adds an opt-in `autoplay` flag to slideshow slides: when the presenter lands
on a video slide, its `<video>` plays from the start. The slideshow still
holds and never auto-advances — the presenter clicks Next when ready. This
covers compositions whose own controls can't be clicked (the player renders
the composition pointer-events:none).

Plumbing (done, tested):
- core: `SlideRef.autoplay?: boolean`, parsed + validated in parseSlideshow
  (a non-boolean autoplay rejects the manifest); carried through resolve.
- controller: optional `PlayerPort.playSceneMedia(sceneId)`, fired only on
  forward `enterSlide` for autoplay slides (not resume/back/sync, so the
  audience — which mirrors the presenter's media events — isn't double-driven).
- component: `playSceneDocumentMedia` reaches the same-origin composition
  iframe, finds the scene's `<video>`, and asserts playback; `stopMedia`
  (already wired on slide change) resets it. An autoplay token cancels a
  pending start when the slide changes.
- tests: controller autoplay behavior + parser flag round-trip/validation
  (131 player + 22 core slideshow tests pass).

KNOWN LIMITATION — runtime media-start needs the player media model (@vance):
On current main the clip<->timeline binding from #1601 keeps every clip synced
and *paused* to the held timeline frame, which wins against playSceneMedia's
play() — so the clip does not actually start on main yet (it does on the
pre-#1601 player). The correct fix is a sanctioned "let this clip free-run
while the timeline holds" path in the player/runtime media controller. Flagging
for Vance to wire the start into the #1601 media model (or rebase onto it) when
back. The plumbing above is the stable surface that hook plugs into.

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

* refactor(slideshow): address code-review on per-slide autoplay

- guard playSceneDocumentMedia behind resolveMode() !== "audience": the
  audience mirrors the presenter's media events, so it must not independently
  drive its own copy of the clip.
- drop the per-enter window pointerdown/keydown "gesture retry" listeners,
  which leaked when muted autoplay succeeded without a gesture. The poll already
  re-asserts play(), so a gesture within the window is picked up next tick.
- stop polling once the clip is advancing across two ticks (was re-asserting
  play() for the full window even after playback was confirmed).
- cancel any in-flight autoplay loop on disconnectedCallback (bump the token).
- split the poll into findSceneVideo + stepAutoplay helpers (keeps each small).
- fix the enterSlide comment: autoplay fires from enterSlide (next/prev/
  goToSlide), not resumeSlide (back/backToMain/syncTo).
- parser: isOptionalBoolean type guard instead of a one-off helper; drop `as`
  assertions in the new controller test.

131 player + 22 core slideshow tests pass; lint/format/typecheck/fallow clean.

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

* docs(slideshow): autoplay skill guidance + address review nits

Addresses review feedback on #1708:
- skill: document per-slide `autoplay` in the slideshow standalone-harness
  reference — when to use it (video is the slide's primary content, its end is
  the advance cue) vs not (background/ambient loops, footage talked over), per
  Vance's guidance, before merge.
- play() rejection is no longer blanket-swallowed: AbortError (timeline-sync
  seek interrupt) and NotAllowedError (gesture-gated autoplay) are expected and
  ignored; any other rejection is surfaced once via console.warn (Via nit 1).
- clarify in the SlideRef.autoplay doc that it plays the scene's FIRST <video>
  (Via nit 2).

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

---------

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

skills/slideshow/references/standalone-harness.md

@@ -109,6 +109,20 @@ For public or user-facing generated projects, make this wrapper the root `index.
 
 `interactive` is required for decks with clickable page content or media controls. Without it, iframe pointer events are disabled by the player shell and a click on the composition can be interpreted as a player play/pause toggle instead of a slide interaction.
 
+### Per-slide `autoplay`
+
+Add `"autoplay": true` to a slide in the island to play that slide's first `<video>` from the start when the presenter lands on it. The slideshow still holds — it never auto-advances — so the presenter clicks Next when ready; autoplay only saves a manual play click into the composition.
+
+```json
+{
+  "sceneId": "cold-open",
+  "autoplay": true,
+  "notes": "Promo plays on enter; click Next when it ends."
+}
+```
+
+Use `autoplay` when the video **is** the slide's primary content and its natural end is the cue to advance — a cold-open promo, a demo clip you let run and then move on from. Do **not** use it for background/ambient loops or for footage the presenter talks over; those should start on the presenter's own cue (a click on the clip's controls via `interactive`), not automatically. One autoplay clip per slide (the first `<video>` in the scene).
+
 ### Presenter media bridge for interactive media
 
 Presenter/audience mode syncs slide position through a deck-scoped `BroadcastChannel`. If the presenter is expected to play, pause, seek, mute, or change rate on media inside the composition, mirror those native media events over the same channel. Keep the media element as the source of truth; do not mirror a custom button's private state.