docs(spf): record realized live refetch policy + LL-HLS change-detection gap

live-presentation-modeling: category [3] refetch policy is realized as the
pure §6.3.4 cadence (mediaPlaylistReloadDelay) + delayedReschedule +
RecurringRunner, fetch scheduling per-track. Record the decided error
semantics — a failed reload's rejection propagates and ends the recurrence;
transient-fetch retry is deferred to network-resilience, kept out of the
cadence — and mark the [3] placement open question decided.

ll-hls-support: note that regular-live change-detection keys off the
full-segment signature, so a parts-only update reads as unchanged; LL-HLS
must fold part progression into change-detection or switch to blocking reload.

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

internal/design/spf/features/ll-hls-support.md

@@ -94,6 +94,15 @@ Things this feature probably forces decisions on, not just additions:
   is the canonical example of a behavior that stays composition-
   variant-agnostic; the LL-HLS reload-loop is its inverse — a
   variant-specific behavior that exists *only* in the LL-HLS engine.
+- **Reload change-detection vs. partial segments** — the regular-live
+  cadence (`mediaPlaylistReloadDelay`) classifies a reload "changed vs.
+  unchanged" — full target vs. ½× per §6.3.4 — from the **full-segment**
+  window signature (`mediaSequence` + segment count), so a parts-only LL-HLS
+  update reads as *unchanged* and would poll at ½ target. The LL-HLS reload-
+  loop must either fold part progression into change-detection (interval-
+  polling variant) or replace interval pacing with response-driven blocking
+  reload, which sidesteps the question. (Regular-live cadence is verified
+  §6.3.4-correct in `reload-cadence.test.ts`; this is the LL-HLS-only gap.)
 - **Variant-decision point — open** — where does the engine commit to
   the LL-HLS variant: adapter-level upfront (consumer opts in), or
   after-first-playlist-parse from `EXT-X-SERVER-CONTROL` flags? The

internal/design/spf/live-presentation-modeling.md

@@ -177,8 +177,20 @@ here.
 target; ½× on an unchanged playlist). This is the category the
 decomposition most insists on isolating: it is a **delivery / scheduling**
 concern, *not* part of the time-*t* content snapshot, even though HLS
-happens to deliver both in the same playlist text. Its model placement
-(per-track vs presentation-level) is an [open question](#open-questions).
+happens to deliver both in the same playlist text.
+
+**Realized** (`feat/spf-hls-live`) as a pure §6.3.4 cadence function
+`mediaPlaylistReloadDelay(current, previous)` (`media/hls/reload-policy.ts`) — full
+target on a changed window, ½× on an unchanged one, `null` once complete — consumed
+by `delayedReschedule` + a `RecurringRunner` (`core/tasks/`) that owns the loop but
+knows nothing about time. Fetch scheduling landed **per-track**: each `resolveXTrack`
+owns its runner (see the [placement open question](#open-questions)). A failed reload
+is **not** retried by the policy — the rejection propagates and ends that track's
+recurrence; transient-fetch resilience (retry/backoff) is a fetch-layer concern
+deferred to [network-resilience](./features/network-resilience.md), kept out of the
+cadence. "Changed" is detected from the full-segment window (`mediaSequence` + segment
+count); a parts-only LL-HLS update is invisible to it, which
+[ll-hls-support](./features/ll-hls-support.md) must extend.
 
 ### [4] Sync anchor — `PROGRAM-DATE-TIME`
 
@@ -228,7 +240,8 @@ behavior are the implementation efforts that consume this.
   model argues for a faithful per-fetch representation. This is the seam
   where this doc meets the parser refactor.
 - **`Presentation`** — add `streamType` ([2a]). Completeness ([2b]) rides the
-  existing `duration` field. Refetch policy ([3]) placement TBD.
+  existing `duration` field. Refetch policy ([3]) realized per-track — each
+  `resolveXTrack` owns a `RecurringRunner` driven by an engine-injected reschedule.
 
 ---
 
@@ -314,16 +327,17 @@ prematurely fix.
 
 ## Open questions
 
-- **[3] Refetch-policy placement — per-track vs presentation-level.** In
+- **[3] Refetch-policy placement — DECIDED (fetch scheduling per-track).** In
   HLS each media playlist (= each `Track`) carries its own
   `TARGETDURATION`; in DASH, `minimumUpdatePeriod` is MPD-level
   (presentation). Per-track bakes an HLS assumption; presentation-level
-  fights HLS's per-playlist reality. Likely answer: a presentation-level
-  concept fed by per-track source data — but decide deliberately. Ties
-  to live-stream-support's per-type reload-coordination question, and to
-  the observation that a not-yet-ended track *must* keep being refetched
-  to stay current, so timeline reconciliation across tracks can't be
-  fully independent even if fetch scheduling is.
+  fights HLS's per-playlist reality. Landed (`feat/spf-hls-live`) as the
+  anticipated split: **fetch scheduling is per-track** — each `resolveXTrack`
+  owns a `RecurringRunner` paced by its own `TARGETDURATION` — while
+  cross-track **timeline reconciliation** stays separate (the parser carries
+  the timeline forward per fetch; `anchorLiveTracks` aligns renditions by PDT).
+  So fetch scheduling is independent without the timeline reconciliation being
+  forced independent, as this question anticipated.
 - **[4] How captured PDT feeds the A/V-sync anchor — DECIDED (anchor source).**
   Resolved in [live-timeline-anchoring](../../decisions/live-timeline-anchoring.md):
   align demuxed audio/video by equal PDT (same real instant), not by