docs(memory): split the frame pipeline out of ai.md into docs/ai

ai.md is the mandatory read every session; at 2,702 lines / ~64k tokens it
costs the whole subsystem history even when the task never touches it. This
is a verbatim move, not a compression - nothing is deleted and each stub is
written from the moved text.

- docs/ai/frame-pipeline.md: 'Frame cadence', 'Per-element composite timing +
  scene dump', 'What the first instrumented take said', 'Render budget',
  'Scene compositor' and 'Live frame pipeline', byte-identical, plus a short
  'Related in ai.md' footer and two stale 'below' pointers corrected.
- ai.md keeps two stubs carrying the invariants that must not be missed
  (overlay on every render path, VideoFrame.Epoch, StripPool threads,
  Release-only render numbers; arrival-stamped rawvideo, non-blocking submit,
  stop order, burned _outputIndex judge) and links the new file.
- docs/ai/index.md + docs/index.md route the split; schema.md inventory lists
  the new tier.
- 4 index-row references and 2 TASKS.md anchors repointed; ai.md's Services
  row too.

Measured: ai.md 63.6k -> 47.1k tokens, 2,702 -> 1,975 lines. Progressive
disclosure per schema.md convention 5. No source touched; no push.

schema.md

@@ -24,6 +24,7 @@ that every other memory file follows. Read `ai.md` first; this file explains
 | `MyMistakes.md` | Per-task failure log **+ RECIPES REGISTRY** (derived-solution rule): one-off solutions / how-tos get recorded here the same session, then grepp'd before ever re-deriving them (see `AGENTS.md` 🔬) | grep first on any "did this before?" wall |
 | `schema.md` | This file: the conventions below | when in doubt |
 | `<dir>/index.md` | Per-directory map (progressive disclosure): what lives there + links | when diving into code |
+| `docs/ai/*.md` | **Split-out subsystems** of `ai.md` — verbatim moves (from 2026-10-26); `ai.md` keeps a short invariant stub + link, and [`docs/ai/index.md`](docs/ai/index.md) routes | when the task touches a moved subsystem |
 | `Views/` | Reserved for Views; currently empty | — |
 
 ## Conventions