Clip aggregate, parse-fidelity validation, trusted-edit perf path

[fuzzzerd]

Summary

Refactors the clip pipeline to a domain-owned aggregate with structured XML→domain parse-fidelity reporting.

• `Clip` aggregate (`src/SharpFM.Model/Clip.cs`) — immutable, lazy-parsed; replaces `FileMakerClip` and evicts INPC from the model layer.
• Strategy table (`src/SharpFM.Model/ClipTypes/`) — compile-time `ClipTypeRegistry.BuiltIns` array with one strategy per `Mac-XM*` format. Adding a new type is a single registration. Collapses the four-site dispatch called out in support additional FileMaker Mac-XM* clipboard types #187.
• `ClipParseResult` / `ClipParseReport` / `ClipParseDiagnostic` (`src/SharpFM.Model/Parsing/`) — every parse produces a structured report classifying losses (`UnknownStep`, `UnknownStepElement`, `RoundTripValueMismatch`, `DroppedNamespace`, etc.) with XPath-style locations.
• `XmlRoundTripDiff` substrate detects unmodeled XML by parse → re-serialize → structural diff, normalising attribute order, namespace prefixes, and whitespace before comparing.
• Visible UX — status-bar parse-fidelity summary and a `!` glyph next to lossy clips in the tree.
• `Clip.FromEditor` trusted-edit path — editor edits skip the strategy parse + diff entirely (the editor's model is the source of truth), avoiding the per-keystroke parse cost on large scripts.
• `FmScript.ToElement` / `FmTable.ToElement` — additive APIs that let the strategies skip the parse-and-prettify round-trip on the diff path.
• `ClipDataExtensions` routes through the registry — plugin-side dispatch consolidated with the host.

Out of scope (tracked in #202)

• `ClipData.ParseReport` (carry the report on the plugin-boundary record).
• `IPluginHost.ValidateClipXml` API.
• Semantic-validation tier (e.g., FileMaker variable name format checks).
• `Clip.Rename` consumer decision.

Test plan

• `dotnet test SharpFM.sln` — 1315 green (1223 SharpFM.Tests + 92 SharpFM.Plugin.Tests).
• Sibling `SharpFM.Productivity` builds and tests pass against this branch.
• Manual: paste a clip with unmodeled attributes from FileMaker; confirm status bar shows the diagnostic count and the tree node carries the `!` glyph.
• Manual: edit a large (~500-step) script; confirm typing is responsive (trusted-edit path skips parse/diff per keystroke).

[github-actions]

Test Results

✔️ Tests 1315 / 1315 - passed in 15.8s
✔️ Coverage 78.75% - passed with 70% threshold
📏 14728 / 17174 lines covered 🌿 4952 / 7816 branches covered
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✏️ updated for commit e078625