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.

Helpers/index.md

@@ -15,8 +15,8 @@ Cross-cutting utilities. See [`schema.md`](../schema.md) for the memory-map conv
 | `InverseBoolToVisibilityConverter.cs` / `NotNullToVisibilityConverter.cs` | XAML value converters for visibility bindings |
 | `AllTrueToVisibilityConverter.cs` | Multi-value AND-gate → Visible; gates the layer-row capture menu items on row-IsBackground + staged-scene-is-Live (TASK 25) |
 | `CameraConflictProbe.cs` | Best-effort diagnostic: when a camera won't start, enumerates running processes known to hold cameras (OBS, Zoom, Teams, NVIDIA Broadcast, browsers, etc.) — Windows doesn't expose "which process has this device" via any public API, so this is a suspect list, not a verdict |
-| `StripPool.cs` | Four DEDICATED background threads at `BelowNormal` priority that split one composite into horizontal row strips (the caller runs a strip too); started lazily, runs inline below 64 rows. UNIT 5 — do NOT demote the ThreadPool from `Parallel.For`, it leaks into unrelated work (see ai.md "Render budget") |
-| `FrameCadence.cs` | Pure arrival-cadence counter (`FrameCadence` + `FrameCadenceSummary.Describe`): frames, gaps, hitches, longest/avg gap for a window of a timestamped stream. Fed by `MediaCaptureFrameSource` (camera stamps vs our arrivals) and `FramePump` (distinct webcam Epochs that reached the output) — UNIT 0, see ai.md "Frame cadence" |
+| `StripPool.cs` | Four DEDICATED background threads at `BelowNormal` priority that split one composite into horizontal row strips (the caller runs a strip too); started lazily, runs inline below 64 rows. UNIT 5 — do NOT demote the ThreadPool from `Parallel.For`, it leaks into unrelated work (see [`docs/ai/frame-pipeline.md`](../docs/ai/frame-pipeline.md) → "Render budget") |
+| `FrameCadence.cs` | Pure arrival-cadence counter (`FrameCadence` + `FrameCadenceSummary.Describe`): frames, gaps, hitches, longest/avg gap for a window of a timestamped stream. Fed by `MediaCaptureFrameSource` (camera stamps vs our arrivals) and `FramePump` (distinct webcam Epochs that reached the output) — UNIT 0, see [`docs/ai/frame-pipeline.md`](../docs/ai/frame-pipeline.md) → "Frame cadence" |
 | `HexToBrushConverter.cs` | A "#RRGGBB" hex string → `SolidColorBrush` (transparent on bad input), so a template can paint a colour swatch straight from the palette. Registered in the theme as `HexToBrush` — the theme paints the colour, never the UI code (TASK 63) |
 | `ClosedBoxTemplateSelector.cs` | An `ItemTemplateSelector` that gives a ComboBox's CLOSED box a different template from its drop-down rows: WPF calls it with the `ContentPresenter`'s templated parent — the `ComboBox` for the closed box, the `ComboBoxItem` for a row. The only way to tell them apart (`SelectionBoxItemTemplate` is read-only and mirrors `ItemTemplate`) — TASK 63 |