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/EffectUnits/AGENT.md

@@ -0,0 +1,53 @@
+# effects module
+
+## Owned files
+
+- Related rack implementation: `../FXProcessor.h`, `../FXProcessor.cpp`
+- Biquad.h
+- Hyper.h, Hyper.cpp
+- Chorus.h, Chorus.cpp
+- Flanger.h, Flanger.cpp
+- Phaser.h, Phaser.cpp
+- Distortion.h, Distortion.cpp
+- EQ.h, EQ.cpp
+- Compressor.h, Compressor.cpp
+- Delay.h, Delay.cpp
+- Reverb.h, Reverb.cpp
+
+## Rules
+
+- Implement the `FXUnit` interface with `prepare`, `reset` and `process (float* l, float* r, int numSamples, const float p[4])`.
+- Process full-wet in place; FXProcessor handles the dry/wet mix.
+- Document what p[0] through p[3] mean for each unit in the header comment.
+- Use `std::vector<float>` for delay lines and size them in `prepare()`.
+- Use `juce::MathConstants<T>::pi` and `twoPi` in delay LFOs, not raw literals.
+- Never allocate or resize a delay line inside `process`.
+- Guard empty delay lines with an early `if (len < 4) return;`.
+
+## IF-THEN
+
+- IF a unit needs a delay line THEN allocate it in `prepare()` at the full required length and only read and write it in `process`.
+- Include the existing rack interface via `../FXProcessor.h`; no source relocation is implied by this guidance.
+
+## Examples
+
+```cpp
+// BAD: reallocates a delay line on the audio thread
+void process (float* l, float* r, int n, const float p[4]) override
+{
+    std::vector<float> delay ((size_t) (sr * 0.05), 0.0f);
+    ...
+}
+```
+
+```cpp
+// GOOD: sized once at prepare time, reused per block
+void prepare (double sampleRate, int) override
+{
+    sr = sampleRate;
+    delayL.assign ((size_t) (sr * 0.05), 0.0f);
+    delayR.assign ((size_t) (sr * 0.05), 0.0f);
+}
+```
+
+This file overrides /AGENT.md where they conflict.