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/Params/AGENTS.md

@@ -0,0 +1,42 @@
+# params module
+
+## Owned files
+
+- Params.h, Params.cpp
+
+## Rules
+
+- Use the `ids::` namespace for every APVTS parameter ID string.
+- Require one `ids::` entry per audio parameter, and one entry only.
+- Reach indexed parameters through the `paramIds::` arrays in `Params.h`, which pair each
+  ID with its slot: `paramIds::oscillators`, `paramIds::envAttack`, `paramIds::lfoRate`,
+  `paramIds::fxType`, `paramIds::macros` and their siblings. Use a single `ids::` entry only
+  when the parameter has no index.
+- Use `enum class` for every enum. Give each a trailing `Count` enumerator.
+- Use `inline constexpr` for parameter ID strings and `k`-prefixed integer constants.
+- Keep all 0..1 to physical unit conversions in the `maps::` namespace.
+- Keep the wavetable name table (`kWavetableNames`) in sync with `kNumWavetables`.
+- Return an empty `juce::String` from enum-to-string switches for unknown values.
+- Never re-declare a parameter ID string literal outside Params.h.
+- Never add a physical unit mapping anywhere but `maps::`.
+
+## IF-THEN
+
+- IF you add an audio parameter THEN add its ID to `ids::` and, when it has physical units, add its mapping to `maps::` before any engine or GUI code reads it.
+- IF a switch converts an enum to a string THEN include a default case that returns an empty string.
+
+## Examples
+
+```cpp
+// BAD: re-declares the ID and drifts from Params.h
+if (auto* p = apvts.getParameter ("f1Cutoff"))
+    return p->getValue();
+```
+
+```cpp
+// GOOD: one source of truth
+if (auto* p = apvts.getParameter (ids::f1Cutoff))
+    return p->getValue();
+```
+
+This file overrides /AGENTS.md where they conflict.