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/Tests/AGENT.md

@@ -0,0 +1,40 @@
+# tests module
+
+## Owned files
+
+- TestMain.cpp
+
+## Rules
+
+- Keep this a headless console harness that drives the real SerumAltAudioProcessor.
+- Check rendered blocks for NaN and inf. Check non-silence over the full preset render, not each block, because note release can legitimately produce silent blocks.
+- Print PASS or FAIL per preset to `std::cout`.
+- Return a non-zero exit code when any check fails.
+- Never show a JUCE alert window.
+- Do not add GUI interaction beyond `juce::ScopedJuceInitialiser_GUI`.
+
+## IF-THEN
+
+- IF a check fails THEN increment the failure count and print the preset name.
+- Build and run natively from the repository root with `./build_harness.sh --run`. Building the `SerumAltTest` target alone does not run checks.
+- The harness-only configuration sets `SERUMALT_BUILD_PLUGIN=OFF` and `SERUMALT_BUILD_TESTS=ON`. Production scripts disable the harness.
+- The harness links the real processor and editor sources but creates no plugin bundles. Keep that build separation when adding checks.
+
+## Examples
+
+```cpp
+// BAD: a failing check does not fail the run
+bool ok = ! hasNaN (buffer) && peak (buffer) > 0.001f;   // result ignored
+return 0;
+```
+
+```cpp
+// GOOD: accumulate failures and exit non-zero
+int failures = 0;
+const bool ok = ! hasNaN (buffer) && peak (buffer) > 0.001f;
+if (! ok) ++failures;
+...
+return failures == 0 ? 0 : 1;
+```
+
+This file overrides /AGENT.md where they conflict.