test(381): golden coverage for Sequential (YAML) playout builder; document Scripted deferral

Extends the #163 PlayoutBuildGoldenTests in-memory net to the Sequential (YAML)
builder: a committed fixture (Goldens/Fixtures/sequential-schedule.yml) with two
`count: 2` instructions over one chronological collection, built via
SequentialPlayoutBuilder over the pinned window. The count/all/duration handlers
do UTC-only arithmetic off the caller-supplied start, so the case is
TZ-independent (passes, not skips, under a non-UTC TZ) and needs no Assume guard.
Non-vacuity: a fixture count tweak flips the golden + the contiguity assertion.

Scripted is deliberately excluded from the golden net — ScriptedPlayoutBuilder
shells out via Cli.Wrap to an external process that drives SchedulingEngine over
HTTP, which no in-memory golden can characterize. Recorded as the Done-when
"documented decision" arm in docs/decisions.md
(testing.scripted-playout-golden-deferred) + docs/testing.md; the scripted
integration harness is tracked as follow-up #563.

fixes #381

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/testing.md

@@ -8,7 +8,7 @@ before adding tests, not just `docs/contributing.md` §8 (which now just points
 | Project | Covers | Notes |
 |---|---|---|
 | `ErsatzTV.Tests` | API controllers + MediatR handlers | In-memory SQLite fixture: a shared `SqliteConnection("Data Source=:memory:;Foreign Keys=False")` kept open + `EnsureCreatedAsync()` (**not** full migration replay) + `PRAGMA foreign_keys=OFF`, then seed; a tiny `IDbContextFactory` wraps `new TvContext(...)`. 828 tests currently. |
-| `ErsatzTV.Core.Tests` | Domain logic, scheduling, IPTV/XMLTV generation | References `ErsatzTV.Application` directly — there is no separate `Application.Tests` project. 542 tests + 1 skipped under `TZ=UTC` (the Block playout golden additionally skips under a non-UTC `TZ`; see Golden-file nets). |
+| `ErsatzTV.Core.Tests` | Domain logic, scheduling, IPTV/XMLTV generation | References `ErsatzTV.Application` directly — there is no separate `Application.Tests` project. 543 tests + 1 skipped under `TZ=UTC` (the Block playout golden additionally skips under a non-UTC `TZ`; see Golden-file nets). |
 | `ErsatzTV.Scanner.Tests` | Library scanning: scan handlers, folder scanners, NFO readers | Handler tests substitute the folder scanners + `ILibraryRepository` and assert the resulting repository writes (e.g. `ScanLocalLibraryHandlerTests` pins which `LastScan` levels a scan records — ersatztv#264). Fakes/`Testably` back the file-system-facing scanners. 1471 tests. Additionally contains `Core/FFmpeg/TranscodingTests` — `[Explicit]` + `[Combinatorial]`, so it never runs in CI or a plain `dotnet test` (it needs real ffmpeg/hardware) and contributes 0 to that count; run it by name when touching the transcoding pipeline. |
 | `ErsatzTV.Architecture.Tests` | Layering rules via NetArchTest.eNhancedEdition | Core↛Infra/App/EF; FFmpeg↛all; App↛concrete providers. 5 tests. See `docs/contributing.md` §1. |
 | `ErsatzTV.FFmpeg.Tests` | FFmpeg command construction | Build a pipeline, assert the exact rendered arg string (`PipelineBuilderBaseTests.cs`). |
@@ -23,8 +23,14 @@ Three golden-file suites guard the highest-value, most-subtle output:
 - **Playout build**: `ErsatzTV.Core.Tests/Scheduling/Goldens/PlayoutBuildGoldenTests.cs` (ersatztv#163)
   — env var **`ETV_UPDATE_PLAYOUT_GOLDENS`** (deliberately separate from `ETV_UPDATE_GOLDENS` so
   regenerating one net can't silently rewrite the other). Snapshots the `PlayoutItem`s each builder
-  produces over a pinned build window. Covers the **Classic** (`PlaybackOrder.Chronological`) and
-  **Block** builders; **Sequential (YAML)** + **Scripted** are tracked in ersatztv#381. A third case,
+  produces over a pinned build window. Covers the **Classic** (`PlaybackOrder.Chronological`),
+  **Block**, and **Sequential (YAML)** builders. **Scripted** is deliberately excluded from this net —
+  `ScriptedPlayoutBuilder` shells out to an external process that drives `SchedulingEngine` over HTTP, so
+  no in-memory golden can characterize it; its integration harness is tracked in ersatztv#563 (decision:
+  `testing.scripted-playout-golden-deferred`). The **Sequential** case (`Sequential_yaml`, ersatztv#381)
+  builds from a committed YAML fixture (`Goldens/Fixtures/sequential-schedule.yml`) instead of a
+  `ProgramSchedule`; it is TZ-independent (the `count`/`all`/`duration` handlers do UTC-only arithmetic —
+  it passes, not skips, under a non-UTC `TZ`) so needs no `Assume` guard. A third case,
   `Classic_clock_padded` (ersatztv#77), locks clock-boundary padding: a `FillerMode.Pad` +
   `PadToNearestMinute=15` PostRoll preset snaps content to `:15`, and the test both goldens the output and
   asserts every content item after the first starts on a quarter-hour. Its EPG counterpart is