docs(02-03): complete MP3 encoder and output flag plan

- Create 02-03-SUMMARY.md with TDD flow documentation
- Update STATE.md: advance to last plan, 100% progress, add decisions
- Update ROADMAP.md: phase 02 complete (3/3 plans)
- Mark requirements OUT-01, OUT-02, OUT-03 complete

.planning/STATE.md

@@ -2,14 +2,14 @@
 gsd_state_version: 1.0
 milestone: v1.0
 milestone_name: milestone
-status: Ready to execute
-stopped_at: "Completed 02-02: OscillatorBank and stereo mixer"
-last_updated: "2026-03-26T11:04:17.995Z"
+status: Phase complete — ready for verification
+stopped_at: "Completed 02-03: MP3 encoder and CLI output flag"
+last_updated: "2026-03-26T11:09:30.198Z"
 progress:
   total_phases: 4
-  completed_phases: 1
+  completed_phases: 2
   total_plans: 7
-  completed_plans: 6
+  completed_plans: 7
 ---
 
 # Project State
@@ -52,6 +52,7 @@ Plan: 3 of 3
 | Phase 01-capture-and-classification P04 | 15min | 2 tasks | 2 files |
 | Phase 02 P01 | 15min | 2 tasks | 8 files |
 | Phase 02 P02 | 10min | 2 tasks | 4 files |
+| Phase 02 P03 | 3min | 2 tasks | 3 files |
 
 ## Accumulated Context
 
@@ -79,6 +80,8 @@ Recent decisions affecting current work:
 - [Phase 02]: go-audio/wav excluded — writing PCM bytes directly to LameWriter is simpler (per Claude's Discretion grant in CONTEXT.md)
 - [Phase 02]: GainPerLayer applied in bank.go during mixing to ensure 11 max layers sum to 1.0 (D-10)
 - [Phase 02]: int16 conversion uses float * 32767 to avoid positive overflow at exactly +1.0
+- [Phase 02]: RunSynthesis takes snapshot slice (not channel) enabling zero-packet guard before file creation
+- [Phase 02]: EncodeMP3 and RunSynthesis are separate functions for independent testability
 
 ### Pending Todos
 
@@ -92,6 +95,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-03-26T11:04:17.988Z
-Stopped at: Completed 02-02: OscillatorBank and stereo mixer
+Last session: 2026-03-26T11:09:30.191Z
+Stopped at: Completed 02-03: MP3 encoder and CLI output flag
 Resume file: None