M2 complete: LP5562 engine patterns running autonomously

Add src/pattern/ module with 3 engine patterns:
- Breathe: smooth ramp up/down (~2.5s cycle, 1 engine)
- Heartbeat: double-pulse with long rest (~1.6s cycle, 1 engine)
- RGB cycle: phase-offset breathing via trigger sync (3 engines)

Patterns support both RGBW and 3-channel monochrome LED modes via
LED_MAP configuration. Main loop cycles through all 3 patterns (8s
each) while LP5562 runs them autonomously — MCU just idles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/STATUS.md

@@ -1,6 +1,6 @@
 # xblink Project Status
 
-**Current Milestone**: M2 — Engine Patterns
+**Current Milestone**: M3 — Hardcoded Pattern Library
 **Last Updated**: 2026-03-05
 
 ---
@@ -28,11 +28,11 @@
 
 ### M2: Engine Patterns
 
-- [ ] Build breathing pattern (1 engine, ramp up/down, branch loop)
-- [ ] Build heartbeat pattern (1 engine, fast ramp, slow decay)
-- [ ] Build RGB color cycle (3 engines, trigger sync)
-- [ ] Verify LP5562 runs patterns autonomously (MCU idle loop)
-- [ ] Verify MCU can stop and switch patterns
+- [x] Build breathing pattern (1 engine, ramp up/down, branch loop)
+- [x] Build heartbeat pattern (1 engine, fast ramp, slow decay)
+- [x] Build RGB cycle (3 engines, trigger-synced phase offset)
+- [x] Verify LP5562 runs patterns autonomously (MCU idle loop)
+- [x] Verify MCU can stop and switch patterns (8s cycle between all 3)
 
 ### M3: Hardcoded Pattern Library