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/EffectUnits/AGENTS.md

@@ -2,7 +2,7 @@
 
 ## Owned files
 
-- Related rack implementation: `../FXProcessor.h`, `../FXProcessor.cpp`
+- FXProcessor.h, FXProcessor.cpp
 - Biquad.h
 - Hyper.h, Hyper.cpp
 - Chorus.h, Chorus.cpp
@@ -27,7 +27,7 @@
 ## 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.
+- Include the rack interface via `FXProcessor.h`.
 
 ## Examples