docs: resolve Phase 18 OQ7 seek-index granularity to 0.5s buckets

PLAN.md

@@ -473,8 +473,8 @@ Settings menu** (not a bare app-bar control); OQ2 default → **Opus by default,
 network-awareness); OQ3 remembered → **persisted via the dark-mode seam** (cookie → prerender-read →
 `PersistentComponentState` → client cookie service); OQ4 → **always-on Opus + Backfill-Opus**; OQ5 →
 **Ogg Opus**; OQ6 transcode model → **background job after the file is available, with a visible
-Post-Processing phase on the CMS upload meter.** One new tuning OQ (OQ7: seek-index granularity — recommend
-~1–2 s buckets) is non-blocking.
+Post-Processing phase on the CMS upload meter.** OQ7 (seek-index granularity) → **0.5 s (half-second)
+buckets** (~115 KB index for a 1-hour mix).
 
 **Architectural spine — a derived artifact set + a delivery param + one new decoder + a precomputed
 accurate seek index; leaf implementations only, zero changes to existing format code (the strong OCP
@@ -496,8 +496,8 @@ path on browsers that can't decode it.
 byte-offset seek and rough page interpolation are inadequate for VBR Opus — there is no linear time↔byte
 relationship. The fix is an **accurate transfer function built at transcode time** (the one moment the
 whole encoded stream is walked): a precomputed **seek index** mapping Ogg-page `granulepos` (48 kHz sample
-counts → time) → exact byte offset (recommend ~1–2 s buckets snapped to page starts; ~58 KB for a 1-hour
-mix). The decode **setup header** (`OpusHead`/`OpusTags`, needed to decode any mid-stream slice) is made
+counts → time) → exact byte offset (**0.5 s buckets** snapped to page starts — OQ7; ~7,200 entries ×
+16 bytes ≈ ~115 KB for a 1-hour mix). The decode **setup header** (`OpusHead`/`OpusTags`, needed to decode any mid-stream slice) is made
 available too. Recommended concrete design: **one sidecar artifact per track = `[setup header][seek
 index]`, built at transcode, stored beside the Opus bytes, fetched once on track load**, parsed into
 `OpusSeekData`. Client seek flow: `calculateByteOffset(t)` binary-searches the index for the exact page
@@ -554,8 +554,8 @@ bytes to serve, decode, or seek against until those artifacts exist.
 
 **Dependency shape:** `18.1 → 18.2 → {18.3 ∥ 18.4} → 18.5`; `18.6 ∥` (needs 18.3 for the live toggle);
 18.1 is the only cold-start wave. **Phase-level: 18 precedes Phase 21** (windowed refill consumes the Phase
-18 seek-index resolver). **OQ1–OQ6 RESOLVED (above); OQ7 (seek-index granularity, recommend ~1–2 s buckets)
-is a non-blocking tuning steer.** None block 18.1.
+18 seek-index resolver). **OQ1–OQ7 RESOLVED (above); OQ7 (seek-index granularity) = 0.5 s buckets.** None
+block 18.1.
 
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