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).

Services/RecordingFile.cs

@@ -0,0 +1,27 @@
+namespace ytLive.Services;
+
+/// <summary>
+/// Pure filename logic for local recordings (TASK 18). The user-requested naming
+/// is <c>ty-yyyymmdd-hhmm-hh2mm2.mp4</c>: <c>ty</c> is the brand marker, the first
+/// <c>hhmm</c> is the hour+minute the recording STARTED (saved), and the second
+/// <c>hh2mm2</c> is the hour+minute of the recording's LENGTH. Length is only known
+/// when recording ends, so the file is created with a <c>0000</c> length and renamed
+/// on stop (see <see cref="FinalizeFileName"/>).
+/// </summary>
+public static class RecordingFile
+{
+    /// <summary>A start placeholder — the length is filled in on stop.</summary>
+    public const string StartLengthSuffix = "0000";
+
+    public static string BuildStartName(DateTime start)
+        => $"ty-{start:yyyyMMdd}-{start:HHmm}-{StartLengthSuffix}";
+
+    /// <summary>Final name with the real recording length (fractional hour+minute),
+    /// e.g. 75 minutes → <c>ty-20260829-1405-0115.mp4</c>.</summary>
+    public static string BuildFinalName(DateTime start, TimeSpan length)
+    {
+        var hh2 = (int)length.TotalHours;
+        var mm2 = length.Minutes;
+        return $"ty-{start:yyyyMMdd}-{start:HHmm}-{hh2:00}{mm2:00}";
+    }
+}