[FEATURE] Preset Manager

[jesusaviladev]

Summary

Adds a local preset system that lets users save and recall the current model, IR,
and EQ/volume settings under a named preset.

Closes #252
Closes #428

Motivation

#252 asked for a way to group model, IR, and EQ/volume params under a named
preset that can be saved and recalled. #428 asked for a generic local
persistence layer (JSON file under the app's Application Support / AppData
path) as infrastructure for exactly this kind of feature, plus others
mentioned in that issue (default paths, one-time messages) that this PR
doesn't attempt to solve.

Design decisions

• Local JSON storage, not VST3's native preset system. The plugin also
  ships as a standalone app, where VST3 native presets don't apply, so a
  self-contained format made more sense than coupling to the host's preset
  mechanism.
• PresetManager is fully decoupled from NeuralAmpModeler. It's a new,
  self-contained class that only knows about a plain PresetData struct
  (name, model path, IR path, and a param name -> value map) — it doesn't
  include NeuralAmpModeler.h or know about the parameter enum. Integration
  in NeuralAmpModeler.cpp is limited to two private methods
  (CollectCurrentState() / ApplyPreset()) that translate between the
  plugin's live state and PresetData, using the existing
  GetParam()->Value() / SetParamValue() API.
• Storage location: presets.json under AppSupportPath() (existing
  WDL/iPlug2 helper), so path resolution is already cross-platform without
  extra code.
• Existing dependency reused: uses nlohmann::json, already vendored in
  the repo — no new dependency added.
• Unserialization.cpp is untouched. That file handles state the host
  serializes into the DAW project, which is a separate concern from this
  local persistence layer.
• The UI control follows the same message-passing pattern
  (SendArbitraryMsgFromUI / message tags) already used elsewhere in
  NeuralAmpModeler.cpp, rather than introducing a new pattern.

What's included

• Save / load / delete named presets (model, IR, EQ params, volume params)
• Duplicate name handling (replaces existing preset with the same name)
• Atomic writes with in-memory rollback if a disk write fails
• Cross-platform: updated both the macOS .xcodeproj and the Windows
  .vcxproj (app and vst3 targets) to include the new files

What's intentionally out of scope

• AAX target wasn't updated — I don't have the Avid SDK to build/verify
  it, so I left NeuralAmpModeler-aax.vcxproj untouched rather than guess.
  Happy to add it if someone can verify the AAX build.
• Other [FEATURE] Persisted information #428 use cases (default model/IR paths, one-time messages) — the
  storage format could be extended for these later, but this PR only
  implements what's needed for presets.
• No unit tests yet for PresetManager — it's designed to be testable in
  isolation (the base path is injected via constructor), so this could be a
  quick follow-up if useful.

Testing

• Compiles clean on macOS (Xcode) and Windows.
• Manually verified end-to-end on both platforms: save a preset, close the
  plugin/app, reopen, load the preset, confirm model/IR/EQ/volume values are
  restored correctly.
• Ran the VST3 SDK Validator against the compiled .vst3 on Windows:
  47/47 tests passed, no new/removed/renamed parameters detected.
• Haven't been able to run the Validator on macOS yet — I don't have
  access to a Mac at the moment. The macOS build was compiled and manually
  tested as noted above, just not run through the formal validator. Happy to
  do that once I have access again, or if CI already covers it, that works
  too.

PR Checklist

• Did you format your code using forrmat.bash?
• Does the VST3 plugin pass all of the unit tests in the VST3PluginTestHost? (Download it as part of the VST3 SDK here.)
  • Windows
  • macOS
• Does your PR add, remove, or rename any plugin parameters? If yes...
  • Have you ensured that the plug-in unserializes correctly?
  • Have you ensured that older versions of the plug-in load correctly? (See Unserialization.cpp.)
• Does your PR add or remove any graphical assets? If yes, are they defined in config.h and added in the two required locations in main.rc?mat.bash`]