docs: add two-way message queue to SYNC.md — ML Intern ↔ Claude Code bridge

Adds:
- Partnership diagram explaining what each side relies on the other for
- ML INTERN → CLAUDE CODE section: question queue before writing code
- CLAUDE CODE → ML INTERN section: answers + diagnostics after builds
- First message: Sprint 4 regression explanation so ML intern understands what broke

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

SYNC.md

@@ -1,10 +1,32 @@
 # PluginBridge — Live Sync File
 
+## THE PARTNERSHIP
+
+```
+ML Intern (HuggingFace)                    Claude Code (Mac)
+────────────────────────                   ─────────────────
+Researcher + Coder                         Implementer + Eyes + Hands
+No Mac access                              No HuggingFace access
+Cannot run or test anything                Cannot write research/design
+
+ML Intern relies on Claude Code for:       Claude Code relies on ML Intern for:
+  - Does this API exist in JUCE 8?           - What are you building next?
+  - Does this compile?                       - What files will you touch?
+  - What does the log say?                   - What approach are you taking?
+  - What's on screen right now?             - Why did you choose this design?
+  - Is this behavior a Mac quirk?
+```
+
+**Neither side should push or build without reading what the other wrote last.**
+
+---
+
 > **How this file works:**
-> ML Intern fills `## NEXT PUSH` BEFORE pushing any code.
-> Claude Code reads it, pulls, builds, tests, then fills `## LAST BUILD RESULT`.
-> After a successful build, Claude Code moves the push details to `## SPRINT LOG`.
-> Neither side should push without reading what the other wrote last.
+> 1. ML Intern posts questions in `## ML INTERN → CLAUDE CODE` before writing code
+> 2. Claude Code answers in `## CLAUDE CODE → ML INTERN` (tests on Mac, verifies APIs)
+> 3. ML Intern fills `## NEXT PUSH` with exactly what it's about to push
+> 4. Claude Code pulls, builds, tests, fills `## LAST BUILD RESULT`
+> 5. Repeat
 
 ---
 
@@ -20,6 +42,39 @@
 
 ---
 
+## ML INTERN → CLAUDE CODE (questions before writing code)
+
+> ML Intern: Post questions here BEFORE writing code that touches Mac-specific behavior.
+> Claude Code will test on the Mac and answer below before you proceed.
+> Do not guess — if you're unsure about an API, a path, or a behavior, ask here first.
+
+```
+STATUS: [no pending questions]
+```
+
+---
+
+## CLAUDE CODE → ML INTERN (answers + diagnostics)
+
+> Claude Code posts answers to ML Intern questions here, plus full diagnostics when builds fail.
+> ML Intern: read this section at the start of every coding session.
+
+```
+STATUS: [no pending answers]
+
+Last message (2026-05-17):
+  The Sprint 4 AudioAnalyser regression was caused by PluginBridgeEditor.mm being
+  reverted to the CALayerHost architecture. That approach is documented as a dead end
+  in FAILED-APPROACHES #10 — it compiles and runs but never renders in Ableton.
+  
+  The correct architecture is in-process createEditor() — see loadInProcessPlugin()
+  in PluginBridgeProcessor.cpp. Do not replace or bypass this function.
+  
+  Fixed files restored from git stash. Build is clean. All tests passing.
+```
+
+---
+
 ## NEXT PUSH — ML Intern fills this before every push
 
 > **ML Intern: Before you push anything, fill this section completely.**