docs: add Phase A test playbook stub and plan backlog

docs/superpowers/plans/_NEXT.md

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+# Plan Backlog
+
+Phase A is implemented (tag: `phase-a-complete`). After Phase A merges and CI is green:
+
+1. **Phase B — NDI Interop & Pipeline** — add real P/Invoke shim in `TeamsISO.Engine.NdiInterop`, real `IFrameScaler` against libyuv, `NdiReceiver` and `NdiSender`, `IsoPipeline`, `IsoController`, runtime version probe. Console smoke runner. Integration test suite goes live (Windows + NDI runtime required).
+2. **Phase C — UI & Packaging** — WPF MVVM app on top of the engine. Settings view, participant list, alert banner, system health indicators. WiX MSI installer, release pipeline on tag, About dialog.
+
+Each phase gets its own `YYYY-MM-DD-teamsiso-phase-X-<topic>.md` plan written by `superpowers:writing-plans` once the previous phase is shipped.

docs/test-playbook.md

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+# TeamsISO Manual Test Playbook
+
+This doc grows with each phase. Phase A is unit-test only — nothing to verify against live Teams yet. Phase B will fill in NDI runtime checks; Phase C will add the live-meeting end-to-end checklist.
+
+## Pre-checks (run before each release branch)
+
+- [ ] `dotnet build TeamsISO.sln` succeeds with zero warnings on Windows.
+- [ ] `dotnet build TeamsISO.Linux.slnf` succeeds with zero warnings on Linux/macOS.
+- [ ] `dotnet test TeamsISO.Linux.slnf --filter "Category!=ndi&requires!=ndi"` reports all unit tests passing.
+- [ ] CI run on `main` is green.
+- [ ] Code coverage on `TeamsISO.Engine` is ≥80%.
+
+## Live-meeting checklist (Phase C)
+
+(To be added.)