refactor(source): move every implementation into its module directory

- 36 files move into the module whose AGENTS.md already claimed them:
  Engine/, Synth/, Modulation/, Params/, Plugin/, GUI/, EffectUnits/, Resources/
- Source/engine, modulation, params, plugin and synth become Engine,
  Modulation, Params, Plugin and Synth, matching the directories that already
  hold code
- ExactCompare.h stays at the Source/ root as the one shared header
- SERUMALT_INCLUDE_DIRS adds one include directory per module, so the 31
  parent-relative includes become plain names with no path prefix
- the module AGENTS.md files list owned files instead of describing a move
  that had not happened, and README.md and AGENTS.md describe the real tree

Verified: the harness passes CTest, the Linux VST3 and Standalone link, the
Windows cross-build passes its module checks, each configure exposes only its
own targets, no first-party warning line appears, and the generated Windows
manifest is unchanged.

Source/Synth/AGENTS.md

@@ -0,0 +1,55 @@
+# synth module
+
+## Owned files
+
+- Wavetable.h, Wavetable.cpp
+- Oscillator.h, Oscillator.cpp
+- SubOscillator.h, SubOscillator.cpp
+- NoiseOscillator.h, NoiseOscillator.cpp
+- Envelope.h, Envelope.cpp
+- LFO.h, LFO.cpp
+- Filter.h, Filter.cpp
+- FilterBank.h, FilterBank.cpp
+- SynthVoice.h, SynthVoice.cpp
+
+## Rules
+
+- Use `noexcept` on every per-sample DSP path and cheap getter.
+- Use `std::array` for fixed-size state and `std::vector<float>` for delay lines and table buffers.
+- Size every buffer and delay line in `prepare()`, never in `process` or `render`.
+- Clamp filter and envelope state so extreme settings cannot produce NaN or inf.
+- Use `juce::MathConstants<T>::pi` and `twoPi` instead of raw pi literals.
+- Use `(size_t)` casts when indexing containers with int loop variables.
+- Pass global per-block state into voices as a `const RenderContext&`.
+- Never allocate memory in the audio path.
+- Keep oscillator phase when changing held-note parameters. LFO phase offsets are separate from the running phase.
+
+## IF-THEN
+
+- Wavetables are prebuilt during Engine construction. Audio and GUI code read them without triggering generation.
+- IF a parameter is read every block THEN snapshot it into a struct (`OscParams`, `RenderContext`) instead of reading the APVTS per sample.
+
+## Examples
+
+```cpp
+// BAD: lazy table build reached from the render path allocates on the audio thread
+const Wavetable& wt = library.getTable (wave);
+```
+
+```cpp
+// GOOD: build once in the Engine constructor, then only read const references
+library.prebuild();                 // in the Engine constructor
+const Wavetable& wt = library.getTable (wave);   // lookup, no allocation
+```
+
+```cpp
+// BAD: raw pi literal duplicated across files
+const double step = 6.28318530717958647692 * freq / sr;
+```
+
+```cpp
+// GOOD: single source
+const double step = juce::MathConstants<double>::twoPi * freq / sr;
+```
+
+This file overrides /AGENTS.md where they conflict.