docs(02-02): complete OscillatorBank and stereo mixer plan

- Created 02-02-SUMMARY.md with plan results and 14 test descriptions
- Updated STATE.md: advanced to plan 3 of 3, recorded metrics and decisions
- Updated ROADMAP.md: phase 02 showing 2/3 summaries complete
- Marked SYNTH-03 complete in REQUIREMENTS.md

.planning/REQUIREMENTS.md

@@ -27,7 +27,7 @@ Requirements for initial release. Each maps to roadmap phases.
 
 - [x] **SYNTH-01**: Each traffic class generates a distinct ambient/drone layer (layered sine/harmonic waves)
 - [x] **SYNTH-02**: Drone layer amplitudes evolve over time windows based on traffic volume per class
-- [ ] **SYNTH-03**: Multiple drone layers are mixed into a single coherent audio stream without distortion
+- [x] **SYNTH-03**: Multiple drone layers are mixed into a single coherent audio stream without distortion
 
 ### Output
 
@@ -78,7 +78,7 @@ Which phases cover which requirements. Updated during roadmap creation.
 | CLAS-04 | Phase 1 | Complete |
 | SYNTH-01 | Phase 2 | Complete |
 | SYNTH-02 | Phase 2 | Complete |
-| SYNTH-03 | Phase 2 | Pending |
+| SYNTH-03 | Phase 2 | Complete |
 | OUT-01 | Phase 2 | Pending |
 | OUT-02 | Phase 2 | Pending |
 | OUT-03 | Phase 2 | Pending |

.planning/ROADMAP.md

@@ -47,11 +47,11 @@ Plans:
   3. Drone layer amplitude rises and falls with traffic volume over time — sustained traffic sounds louder, quiet periods fade
   4. User can specify output path via `-o` flag; it defaults to `netsynth-<timestamp>.mp3` when omitted
   5. An empty (zero-packet) input produces a clear error message instead of a corrupt or zero-byte MP3
-**Plans:** 0/3 plans executed
+**Plans:** 2/3 plans executed
 
 Plans:
-- [ ] 02-01-PLAN.md — Environment setup (gcc, ffprobe, go-lame), synth config table, oscillator, EMA layer with tests
+- [x] 02-01-PLAN.md — Environment setup (gcc, ffprobe, go-lame), synth config table, oscillator, EMA layer with tests
-- [ ] 02-02-PLAN.md — Stereo mixer (constant-power panning), OscillatorBank multi-layer rendering with tests
+- [x] 02-02-PLAN.md — Stereo mixer (constant-power panning), OscillatorBank multi-layer rendering with tests
 - [ ] 02-03-PLAN.md — MP3 encoder package, zero-packet guard, -o CLI flag, ffprobe integration test
 
 ### Phase 3: Pipeline Integration and MVP
@@ -82,6 +82,6 @@ Phases execute in numeric order: 1 → 2 → 3 → 4
 | Phase | Plans Complete | Status | Completed |
 |-------|----------------|--------|-----------|
 | 1. Capture and Classification | 4/4 | Complete   | 2026-03-25 |
-| 2. Audio Synthesis Engine | 0/3 | Planned    |  |
+| 2. Audio Synthesis Engine | 2/3 | In Progress|  |
 | 3. Pipeline Integration and MVP | 0/? | Not started | - |
 | 4. Power User Features | 0/? | Not started | - |

.planning/STATE.md

@@ -3,13 +3,13 @@ gsd_state_version: 1.0
 milestone: v1.0
 milestone_name: milestone
 status: Ready to execute
-stopped_at: "Completed 02-01: synth config, oscillator, and layer"
+stopped_at: "Completed 02-02: OscillatorBank and stereo mixer"
-last_updated: "2026-03-26T10:59:51.259Z"
+last_updated: "2026-03-26T11:04:17.995Z"
 progress:
   total_phases: 4
   completed_phases: 1
   total_plans: 7
-  completed_plans: 4
+  completed_plans: 6
 ---
 
 # Project State
@@ -24,7 +24,7 @@ See: .planning/PROJECT.md (updated 2026-03-24)
 ## Current Position
 
 Phase: 02 (audio-synthesis-engine) — EXECUTING
-Plan: 2 of 3
+Plan: 3 of 3
 
 ## Performance Metrics
 
@@ -51,6 +51,7 @@ Plan: 2 of 3
 | Phase 01-capture-and-classification P02 | 3min | 1 tasks | 4 files |
 | 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 |
 
 ## Accumulated Context
 
@@ -76,6 +77,8 @@ Recent decisions affecting current work:
 - [Phase 01-04]: Three-stage pipeline: capture -> classify goroutine -> aggregate via buffered channels; 1024-buffered classified channel absorbs burst processing
 - [Phase 02]: go-lame v0.0.9 used for MP3 encoding (embedded LAME C source, no system library needed)
 - [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
 
 ### Pending Todos
 
@@ -89,6 +92,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-03-26T10:59:51.252Z
+Last session: 2026-03-26T11:04:17.988Z
-Stopped at: Completed 02-01: synth config, oscillator, and layer
+Stopped at: Completed 02-02: OscillatorBank and stereo mixer
 Resume file: None

.planning/phases/02-audio-synthesis-engine/02-02-SUMMARY.md

@@ -0,0 +1,91 @@
+---
+phase: 02-audio-synthesis-engine
+plan: "02"
+subsystem: synth
+tags: [audio-synthesis, stereo-mixing, oscillator-bank, pcm]
+dependency_graph:
+  requires: ["02-01"]
+  provides: ["02-03"]
+  affects: ["synth/bank.go", "synth/mixer.go"]
+tech_stack:
+  added: []
+  patterns:
+    - constant-power (equal-power) pan law via cos/sin mapping
+    - GainPerLayer (1/11) fixed-gain mixing to prevent clipping by construction
+    - WindowSnapshot -> OscillatorBank.RenderWindow -> [][2]float64 pipeline
+    - interleaved little-endian int16 PCM bytes for go-lame encoder
+key_files:
+  created:
+    - synth/bank.go
+    - synth/bank_test.go
+    - synth/mixer.go
+    - synth/mixer_test.go
+  modified: []
+decisions:
+  - "GainPerLayer applied in bank.go during mixing (not in layer.go) per D-10 — ensures 11 max layers sum to 1.0"
+  - "PanGains uses angle = (p+1)/2 * pi/2 mapping so p=-1 -> cos(0)=1.0/sin(0)=0.0 and p=1 -> cos(pi/2)=0.0/sin(pi/2)=1.0"
+  - "int16 conversion: float * 32767 (not 32768) avoids positive overflow at exactly +1.0"
+metrics:
+  duration: "10min"
+  completed: "2026-03-26"
+  tasks: 2
+  files: 4
+---
+
+# Phase 02 Plan 02: OscillatorBank and Stereo Mixer Summary
+
+**One-liner:** OscillatorBank with 11 EMA-smoothed layers driven by WindowSnapshot, mixed with 1/11 fixed gain and constant-power stereo panning into interleaved int16 PCM bytes.
+
+## What Was Built
+
+### synth/mixer.go
+
+Provides two core utilities:
+
+- `PanGains(p float64) (gainL, gainR float64)` — constant-power pan law using cos/sin. Maps [-1, 1] pan position to L/R gain pair where `gainL^2 + gainR^2 = 1.0` at all positions.
+- `StereoFramesToInt16Bytes(frames [][2]float64) []byte` — converts stereo float64 frames to interleaved little-endian int16 bytes for go-lame's `Write()` method. Clamps values to [-1.0, 1.0] before conversion to prevent int16 overflow.
+
+### synth/bank.go
+
+Provides the core synthesis engine:
+
+- `NewBank(tau float64) *OscillatorBank` — creates 11 layers, one per TrafficClass, each initialized from `ClassFreqConfigs`.
+- `RenderWindow(snap classify.WindowSnapshot) [][2]float64` — updates all layer amplitude targets from the snapshot's packet counts, then renders exactly `SamplesPerWindow` (22050) stereo frames. Each layer contributes `GainPerLayer` (1/11) of headroom, so even 11 simultaneously maxed layers cannot exceed [-1.0, 1.0].
+
+## Tests Added (14 new tests)
+
+**mixer_test.go (7 tests):**
+- `TestPanGainsCenter` — verifies center position yields sqrt(2)/2 for both channels
+- `TestPanGainsFullLeft` — gainL=1.0, gainR=0.0 at p=-1.0
+- `TestPanGainsFullRight` — gainL=0.0, gainR=1.0 at p=1.0
+- `TestPanGainsPowerPreserved` — gainL^2 + gainR^2 = 1.0 for 9 pan values
+- `TestStereoFramesToInt16Bytes` — 1.0/-1.0 maps to 32767/-32767 LE int16
+- `TestStereoFramesToInt16BytesZero` — 0.0/0.0 maps to 4 zero bytes
+- `TestClampPreventsOverflow` — 2.0/-2.0 clamps to 32767/-32767 (no wrap-around)
+
+**bank_test.go (7 tests):**
+- `TestNewBankHas11Layers` — bank has exactly 11 layers, one per AllClasses()
+- `TestRenderWindowOutputLength` — returns exactly 22050 frames
+- `TestRenderWindowSilentWhenNoTraffic` — all-zero frames when no class ever seen
+- `TestRenderWindowNonZeroWithTraffic` — ICMP count=100 produces non-zero output
+- `TestMixerNoClip` — 11 classes at max count, 10 windows rendered, no |L| or |R| > 1.0
+- `TestStereoPan` — ClassDHCP (pan=-0.75) yields rmsL > rmsR (wide-left verified)
+- `TestMultipleWindowsEMAConvergence` — RMS grows over 5 windows with same snapshot
+
+## Deviations from Plan
+
+None — plan executed exactly as written.
+
+## Self-Check
+
+Files created:
+- synth/mixer.go: EXISTS
+- synth/mixer_test.go: EXISTS
+- synth/bank.go: EXISTS
+- synth/bank_test.go: EXISTS
+
+Commits:
+- c9794cd: feat(02-02): stereo mixer utilities with constant-power pan law
+- 23dcfdb: feat(02-02): OscillatorBank multi-layer rendering from WindowSnapshot
+
+## Self-Check: PASSED