feat(spf): architecture reactors

.claude/plans/spf/actor-migration-assessment.md

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+# Actor Migration Assessment
+
+Assessment of migrating SPF's segment-related actors to the `createMachineActor` factory.
+Written after completing the text track Actor/Reactor spike. **Partially stale** — see
+inline notes for what has been completed.
+
+---
+
+## Background
+
+The text track spike produced reference implementations using three actor patterns:
+
+- `TextTracksActor` — uses `createTransitionActor` (reducer-style, no FSM)
+- `TextTrackSegmentLoaderActor` — manual `CallbackActor` (lightweight, no framework)
+- Reactors (`syncTextTracks`, `loadTextTrackCues`) — use `createMachineReactor`
+
+The segment-loading layer had three actors to consider:
+`SegmentLoaderActor`, `SourceBufferActor`, and `loadSegments` (Reactor).
+
+---
+
+## SegmentLoaderActor ✅ (Completed)
+
+**File:** `packages/spf/src/dom/features/segment-loader-actor.ts`
+
+Migrated to `createMachineActor` with `idle`/`loading` states, `onSettled: 'idle'`,
+and continue/preempt logic. The continue case uses `SerialRunner.abortPending()` (added
+during migration) — drops queued tasks without touching the in-flight task. In-flight
+tracking (`inFlightInitTrackId`, `inFlightSegmentId`) lives in actor context via
+`setContext`/`getContext`.
+
+---
+
+## SourceBufferActor ✅ (Completed)
+
+**File:** `packages/spf/src/dom/media/source-buffer-actor.ts`
+
+Migrated to `createMachineActor` with `idle`/`updating` states, `onSettled: 'idle'`,
+and a `cancel` message in the `updating` state. The four originally-identified blockers
+were resolved:
+
+1. **Awaitable send** — `SegmentLoaderActor` now observes completion via
+   `waitForIdle(snapshot, signal)` rather than awaiting `send()` directly.
+2. **Context as task output** — tasks return new context; `.then(setContext)` commits it.
+   `getContext` threading ensures each task reads the context committed by the previous task.
+3. **`batch()`** — implemented as a message type; handler iterates and schedules all tasks.
+4. **Partial updates** — `setContext()` called mid-task for streaming segment progress.
+
+---
+
+## loadSegments (Reactor)
+
+**File:** `packages/spf/src/dom/features/load-segments.ts`
+
+Currently a function with signals/effects inline — it is the Reactor layer that
+bridges state → `SegmentLoaderActor` messages. Not itself an actor.
+
+Could be rewritten as a class-based Reactor if a `createMachineReactor` pattern is established
+(per the primitives.md design). Lower priority than the actor migrations; natural
+follow-on after `SegmentLoaderActor` is migrated.
+
+---
+
+## Status
+
+- ✅ `SegmentLoaderActor` — migrated to `createMachineActor` (with `abortPending()` instead of `replaceQueue()`)
+- ✅ `SourceBufferActor` — migrated to `createMachineActor`
+- ⬜ `loadSegments` — still function-based; natural follow-on as a `createMachineReactor` migration