docs: add agent guidance and historical audit report

Add root AGENT.md with project conventions, build verification steps,
source layout, style rules, and real-time/concurrency requirements.
Add per-module AGENT.md files for each existing and proposed source
subdirectory. Add AUDIT_REPORT.md as a historical Phase 1 snapshot
documenting memory management, error handling, concurrency model,
naming conventions, and anti-pattern catalog.

Source/engine/AGENT.md

@@ -0,0 +1,48 @@
+# engine module
+
+## Related source files
+
+This directory contains guidance; the implementation remains in `../Engine.h`
+and `../Engine.cpp`. The preparation rules below are requirements for changes;
+current buffer resizing and lazy wavetable issues are recorded in the root AGENT.md.
+
+## Rules
+
+- Own the voice pool, LFOs, wavetable library, FX rack, matrix and macros by value.
+- Read every APVTS value through the null-guarding `v()` helper.
+- Build the `RenderContext` once per block and pass it to voices as `const&`.
+- Size `mixBuffer` once in `prepare()` and only `clear()` it per block.
+- Prebuild wavetables in `prepare()` before the first render.
+- Clamp the final output with the soft limiter.
+- Never call `setSize` on any buffer inside `processBlock`.
+
+## IF-THEN
+
+- IF a per-block buffer is needed THEN allocate it in `prepare()` and clear it per block.
+- IF a module needs the matrix, macros or wavetables THEN go through Engine accessors rather than reaching into another module.
+
+## Examples
+
+```cpp
+// BAD: per-callback buffer reconfiguration on the audio thread
+void processBlock (...)
+{
+    mixBuffer.setSize (2, n, false, false, true);
+    ...
+}
+```
+
+```cpp
+// GOOD: size once, clear per block
+void prepare (double sr, int blockSize)
+{
+    mixBuffer.setSize (2, blockSize, false, false, true);
+}
+void processBlock (...)
+{
+    mixBuffer.clear();
+    ...
+}
+```
+
+This file overrides /AGENT.md where they conflict.