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/Modulation/ModulationMatrix.cpp

@@ -0,0 +1,76 @@
+#include "ModulationMatrix.h"
+
+namespace serum
+{
+
+bool ModulationMatrix::addConnection (ModSource source, ModTarget target, float depth, bool bipolar)
+{
+    if ((int) source < 0 || (int) source >= kNumModSources
+        || (int) target < 0 || (int) target >= kNumModTargets
+        || ! std::isfinite (depth) || (int) connections.size() >= kMaxConnections)
+        return false;
+    connections.push_back ({ source, target, juce::jlimit (-1.0f, 1.0f, depth), bipolar });
+    return true;
+}
+
+void ModulationMatrix::removeConnection (int index)
+{
+    if (index >= 0 && index < (int) connections.size())
+        connections.erase (connections.begin() + index);
+}
+
+void ModulationMatrix::removeAllWithTarget (ModTarget target)
+{
+    connections.erase (std::remove_if (connections.begin(), connections.end(),
+        [target] (const ModConnection& c) { return c.target == target; }),
+        connections.end());
+}
+
+juce::ValueTree ModulationMatrix::toValueTree() const
+{
+    ModulationMatrix validated;
+    for (const auto& c : connections)
+    {
+        if (validated.size() >= kMaxConnections)
+            break;
+        if (! validated.addConnection (c.source, c.target, c.depth, c.bipolar))
+            continue;
+    }
+    juce::ValueTree tree ("MODMATRIX");
+    for (const auto& c : validated.connections)
+    {
+        juce::ValueTree con ("CONNECTION");
+        con.setProperty ("source",   modSourceToString (c.source), nullptr);
+        con.setProperty ("target",   modTargetToString (c.target), nullptr);
+        con.setProperty ("depth",    c.depth, nullptr);
+        con.setProperty ("bipolar",  c.bipolar, nullptr);
+        tree.appendChild (con, nullptr);
+    }
+    return tree;
+}
+
+void ModulationMatrix::fromValueTree (const juce::ValueTree& tree)
+{
+    connections.clear();
+    if (! tree.hasType ("MODMATRIX"))
+        return;
+
+    for (const auto& con : tree)
+    {
+        if ((int) connections.size() >= kMaxConnections)
+            break;
+        if (! con.hasType ("CONNECTION"))
+            continue;
+        const auto sourceName = con.getProperty ("source").toString();
+        const auto source = modSourceFromString (sourceName);
+        if (sourceName.isEmpty() || modSourceToString (source) != sourceName)
+            continue;
+        if (! addConnection (source,
+                             modTargetFromString (con.getProperty ("target").toString()),
+                             (float) con.getProperty ("depth", 0.0),
+                             (bool) con.getProperty ("bipolar", false)))
+            continue;
+    }
+}
+
+} // namespace serum