docs(02-01): complete audio engine plan 01 — MPVEngine, SUMMARY, STATE, ROADMAP updated

- Create 02-01-SUMMARY.md with task commits, decisions, self-check
- Advance plan counter to plan 2 of 3
- Update progress to 71% (5/7 plans complete)
- Add 3 key decisions to STATE.md
- Mark AUD-01, AUD-03, INF-03 complete in REQUIREMENTS.md
- Update ROADMAP.md phase 2 progress

.planning/STATE.md

@@ -3,14 +3,14 @@ gsd_state_version: 1.0
 milestone: v1.0
 milestone_name: milestone
 status: executing
-stopped_at: Completed 01-foundation/01-04-PLAN.md
-last_updated: "2026-04-06T12:24:20.367Z"
+stopped_at: Completed 02-audio-engine/02-01-PLAN.md
+last_updated: "2026-04-06T13:04:49.667Z"
 last_activity: 2026-04-06
 progress:
   total_phases: 7
   completed_phases: 1
-  total_plans: 4
-  completed_plans: 4
+  total_plans: 7
+  completed_plans: 5
   percent: 0
 ---
 
@@ -21,12 +21,12 @@ progress:
 See: .planning/PROJECT.md (updated 2026-04-05)
 
 **Core value:** Load a song, build a synchronized light show, play it back live — with precise control over every effect on every device.
-**Current focus:** Phase 01 — foundation
+**Current focus:** Phase 02 — audio-engine
 
 ## Current Position
 
-Phase: 2
-Plan: Not started
+Phase: 02 (audio-engine) — EXECUTING
+Plan: 2 of 3
 Status: Ready to execute
 Last activity: 2026-04-06
 
@@ -55,6 +55,7 @@ Progress: [░░░░░░░░░░] 0%
 | Phase 01-foundation P01 | 4 | 2 tasks | 23 files |
 | Phase 01-foundation P03 | 6 | 1 tasks | 9 files |
 | Phase 01 P04 | 2 | 2 tasks | 3 files |
+| Phase 02-audio-engine P01 | 525603 | 2 tasks | 5 files |
 
 ## Accumulated Context
 
@@ -74,6 +75,9 @@ Recent decisions affecting current work:
 - [Phase 01-foundation]: API collection routes use empty-string paths ('') not '/' — canonical /api/devices without redirect
 - [Phase 01]: window.removeDevice export required for ES module onclick compatibility — functions in type=module scripts are not global
 - [Phase 01]: CSS accent variable canonical naming: --accent is source of truth, aliases are text-accent and border-bright
+- [Phase 02-audio-engine]: MPV_AO env var controls audio output — null default for Docker server mode, overridable for local dev
+- [Phase 02-audio-engine]: 10Hz polling via time.sleep(0.1) over observe_property — observe_property fires per audio frame overwhelming asyncio
+- [Phase 02-audio-engine]: apply_timer_fix() as Linux no-op — timeBeginPeriod is Windows-only; INF-03 satisfied as platform-guarded stub
 
 ### Pending Todos
 
@@ -87,8 +91,8 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-04-05T20:14:32.598Z
-Stopped at: Completed 01-foundation/01-04-PLAN.md
+Last session: 2026-04-06T13:04:49.658Z
+Stopped at: Completed 02-audio-engine/02-01-PLAN.md
 Resume file: None
 
 Next action: `/gsd:plan-phase 1`