vkconfig: Add layer manifest validation multithreading

[herlience]

Fixes #2659

Splits manifest loading into a parallel parse phase (file read +
JSON parse) and a sequential apply phase (validation + inserting
into available_layers), since the latter touches shared state.

Added:

• ParseLayerFile(): reads a manifest file and parses its JSON.
  No shared state, so safe to run on multiple threads at once.
• ApplyParsedLayer(): takes already-parsed JSON, validates it,
  and inserts/updates it in available_layers. Kept sequential
  because it mutates shared data and depends on ordering.

Removed:

• LoadLayers(): did file read + parse + validate + insert all
  in one function, entirely sequentially. Replaced by the two
  functions above so the expensive I/O/parse part can run in
  parallel.

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