TASK 18: local (record-only) recording pipeline + top-bar pills/light/Start-End button UX

Encoder dual-output: StreamEnabled/RecordEnabled/RecordPath options; per-output
ffmpeg block (stream ->flv, record ->mp4); throw unless at least one output.

RecordingFile: auto-name ty-<yyyymmdd>-<start hhmm>-0000.mp4, rename-on-stop to
ty-...-<len hhmmss>.mp4 with numeric-suffix on collision. LayoutStore persists
record folder. MainViewModel: REC/ON-AIR pills, status light, single Start<->End
button (replaces Start/Stop variants), StopStream no longer signs out, explicit
session timer. About overlay widened. Sign In label + spacing before Start.

Tests: 244 passed / 246 (2 pre-existing: AudioPipeline gains-mute, RoundClip).
Build 0 warnings. Scope check clean.

Memory-map correction (derived-solution rule, 2026-08-29): image-shrink recipe was
never recorded when first done, so it was re-derived. Added Derived-solution/recipes
rule to AGENTS.md; MyMistakes.md now a permanent recipes registry (records the
verified WPF-imaging shrink recipe); schema.md rows MyMistakes as registry; ai.md
points to the rule. README hero image replaced: 3.2MB screenshot -> 1400x794 ->
188KB docs/ytLlive-preview.jpg (JPEG q82).

AGENTS.md

@@ -53,6 +53,15 @@ at a problem.
 - **The first time a fact costs a hunt (secrets path, DB path, port, recovery
   source), record it** in `ai.md`/indexes/`HANDOFF.md` so the next session never
   re-hunts it.
+- **Derived-solution rule (record once, then grep):** the moment you work out a
+  reusable solution — a recipe, workaround, or how-to (e.g. how to shrink and
+  re-encode an image for the README) — write it into `MyMistakes.md` **in the same
+  session**, before you finish the task. And always **grep `MyMistakes.md` first**
+  when you hit a "I've done this before but have to figure it out again" wall. A
+  solution recorded once ends the re-derivation loop; an un-recorded solution is a
+  guarantee you will re-derive it and make the user sit through it again. This is
+  the fix for the image-shrink incident (2026-08-29): the recipe was never written
+  down, so it had to be worked out from scratch a second time.
 - **Follow existing conventions** — MVVM, `RelayCommand` for actions,
   `ViewModelBase.SetProperty<T>()`, all styles in `Themes/Controls.xaml`
   (merged once in `App.xaml`; never duplicate per-window).