Plugin-host parse-fidelity API + semantic-validation tier

[fuzzzerd]

Closes #202.

Bundles the four follow-up items from #202:

1. ClipParseReport on ClipData — plugins can now read fidelity diagnostics without re-parsing via the new ClipData.ParseReport init-only property. PluginHost populates it at every new ClipData site from the live Clip aggregate's cached parse.

2. IPluginHost.ValidateClipXml(clipType, xml) → ClipParseReport — pure pre-flight validator routed through ClipTypeRegistry.For(clipType).Parse(xml).Report. No mutation, no UI-thread gating. Lets external callers inspect lossiness before pushing XML through UpdateClipXml / UpdateSelectedClipXml.

3. Semantic-validator framework (no rules) — new SharpFM.Model.Validation namespace with IClipSemanticValidator and SemanticValidatorRegistry. Wired into Clip.FromXml and Clip.FromEditor so diagnostics fold into a separate ClipParseReport.SemanticDiagnostics list. Structural and semantic axes stay distinct — IsLossless keeps its existing meaning, and a new IsSemanticallyValid mirrors it on the semantic side. Ships with BuiltIns = []; concrete rules land in follow-ups.

4. Clip.Rename wired to a UI command — new "Rename..." context-menu item on the clip tree, backed by MainWindowViewModel.RenameSelectedClip. Adds the SharpFM.Dialogs namespace (IInputPrompt + InputDialog) and fills in the previously-stubbed IPluginHost.ShowInputDialogAsync via the same plumbing.

Also fixes a pre-existing UX gap: right-clicking a clip in the tree now promotes that node into the active selection before the context menu opens, so Rename / Delete / Copy operate on the right-clicked clip rather than whatever was previously focused.

1345 tests pass (dotnet test SharpFM.sln).

[github-actions]

Test Results

✔️ Tests 1345 / 1345 - passed in 16.2s
✔️ Coverage 78.9% - passed with 70% threshold
📏 14810 / 17242 lines covered 🌿 4993 / 7858 branches covered
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✏️ updated for commit 7a3cf3e