docs(20-02): complete OGG Vorbis tag writer tests plan

.planning/phases/20-ogg-vorbis-tag-writer/20-02-SUMMARY.md

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+---
+phase: 20-ogg-vorbis-tag-writer
+plan: 02
+subsystem: tagwriter
+tags: [ogg, vorbis, testing, round-trip, crc32, metadata, cover-art, dhowden-tag]
+
+# Dependency graph
+requires:
+  - phase: 20-ogg-vorbis-tag-writer
+    provides: writeOggTags, parseOggPages, oggCRC, pipeline integration
+provides:
+  - Comprehensive round-trip tests for OGG Vorbis tag writer (OGG-01 through OGG-06)
+  - Programmatic OGG Vorbis test fixture builder
+  - CRC32 validation against known vectors
+affects: [tagwriter]
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns: [programmatic OGG Vorbis fixture generation, CRC32 known-vector validation]
+
+key-files:
+  created:
+    - backend/tagwriter/ogg_test.go
+  modified: []
+
+key-decisions:
+  - "Programmatic OGG fixture builder instead of embedded binary — avoids fragile byte literals, uses Plan 01 page structures directly"
+  - "CRC32 validated against independently computed known vectors (OggS→0x5fb0a94f, {1..8}→0x7d0f3681) plus fixture self-consistency"
+
+patterns-established:
+  - "OGG test fixture via createTestOGG: builds identification + comment + setup + audio pages programmatically"
+  - "Audio preservation test: capture audio page data before/after write, assert byte-identical"
+
+requirements-completed: [OGG-01, OGG-02, OGG-03, OGG-04, OGG-05, OGG-06]
+
+# Metrics
+duration: 5min
+completed: 2026-03-19
+---
+
+# Phase 20 Plan 02: OGG Vorbis Tag Writer Tests Summary
+
+**9 round-trip test functions covering all 6 OGG requirements with programmatic fixture builder, CRC32 known-vector validation, and dhowden/tag read-back verification**
+
+## Performance
+
+- **Duration:** 5 min
+- **Started:** 2026-03-19T17:56:41Z
+- **Completed:** 2026-03-19T18:02:38Z
+- **Tasks:** 3
+- **Files modified:** 1
+
+## Accomplishments
+- Programmatic OGG Vorbis test fixture builder (createTestOGG) that constructs valid files from page structures
+- CRC32 validation against known vectors and fixture page self-consistency
+- All 9 text fields round-trip correctly through writeOggTags → metadata.ExtractTags (OGG-01)
+- Cover art embed, replace, and clear via METADATA_BLOCK_PICTURE (OGG-04)
+- Non-edited fields survive partial updates (OGG-02)
+- Audio page data byte-identical after tag write (OGG-03)
+- Atomic safety: corrupt files untouched on failure (OGG-05)
+- Non-Vorbis and multi-stream OGG rejection tested
+- Full tagwriter suite (MP3 + FLAC + WAV + OGG) passes with zero regressions
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: Create OGG test fixture and CRC32 validation** - `246a991` (test)
+2. **Task 2: Write round-trip tests for all OGG requirements** - `f13aa70` (test)
+3. **Task 3: Full test suite verification** - no commit (verification-only, all green)
+
+## Files Created/Modified
+- `backend/tagwriter/ogg_test.go` - 659 lines: test fixture builder, CRC32 validation, 9 round-trip test functions covering all OGG requirements
+
+## Decisions Made
+- Built OGG test fixture programmatically using Plan 01's page structures (buildVorbisIdentPacket, buildHeaderPages, writeOggPage) rather than embedding a pre-generated binary — gives full control and avoids fragile byte literals
+- CRC32 validated with independently computed known vectors plus self-consistency checks on fixture pages
+
+## Deviations from Plan
+
+None - plan executed exactly as written.
+
+## Issues Encountered
+None
+
+## User Setup Required
+None - no external service configuration required.
+
+## Next Phase Readiness
+- Phase 20 (OGG Vorbis tag writer) is complete — implementation + tests both done
+- All format writers now have comprehensive test coverage: MP3, FLAC, WAV, OGG
+- Ready for phase transition to next milestone phase
+
+## Self-Check: PASSED
+
+All files verified on disk, all commits found in git log. ogg_test.go is 659 lines (exceeds 300 minimum).
+
+---
+*Phase: 20-ogg-vorbis-tag-writer*
+*Completed: 2026-03-19*