BookmarkManager carried a byte-for-byte copy of ILSpySettings' private
MutexProtector, both guarding read-modify-write of a config sidecar in the same
settings directory so parallel ILSpy instances fold their edits in turn instead
of clobbering each other. Promote it to a single public type in
ICSharpCode.ILSpyX.Settings so the locking semantics live in one place.
Assisted-by: Claude:claude-opus-4-8:Claude Code
* Move ILSpySettings to ILSpyX
* Make settings file path configurable using a static provider interface
* Move MiscSettings to ILSpyX, rename existing to MiscSettingsVieModel
* Introduce static Load for DecompilerSettings on interface
* Add path provider for ilspycmd parameter scenario
* Allow for saving of MiscSettingsPanel
* Rename DisplaySettings to DisplaySettingsViewModel
* Add SaveDecompilerSettings