refactor: build clip XML via XElement instead of string interpolation

[fuzzzerd]

Summary

Replaces nine string-interpolated XML construction sites with XElement / XAttribute / XCData API calls. The framework handles attribute escaping and CDATA wrapping; manual escaping is no longer needed.

Why

TableClipStrategy.DefaultXml previously interpolated a clip name into an XML attribute via $"<BaseTable name=\"{clipName}\">". A clip named My "favorite" stuff produced malformed XML; a recent fix patched the symptom by calling a hand-rolled XmlHelpers.XmlEscape before interpolation. Auditing for the same anti-pattern surfaced eight more sites: Calculation.ToXml and seven XElement.Parse($"<X><![CDATA[{userText}]]></X>") callsites in FmField.ToXml. All share the same root cause — building structured XML by string concatenation — and the same class of edge-case failure (e.g. ]]> inside calc text would terminate the CDATA early and throw on parse).

Scope

• TableClipStrategy.DefaultXml → new XElement("BaseTable", new XAttribute("name", clipName))
• Calculation.ToXml → new XElement(name, new XCData(Text))
• FmField.ToXml (×7: Calculation, ConstantData, MinimumValue, MaximumValue, StrictValidation, ErrorMessage, AutoEnter-Calculation) → same XCData pattern
• XmlHelpers.XmlEscape deleted (no callers remain)
• IClipTypeStrategy.DefaultXml doc-comment updated to point at XElement rather than at a manual-escape helper

new XCData(text) still can't legally hold the literal sequence ]]> — that's an XML spec limitation, not an API one. The failure mode improves from "silent on-disk corruption" to "throw on save," which is acceptable; FileMaker's own calc grammar can't produce ]]> either.

Tests

All 1424 existing tests still pass. Added 3 new Calculation.RoundTrip_PreservesXmlMetacharacters cases covering <, &, ", ' in calc text. Added one O'Brien Theory row to Table_DefaultXml_EscapesPunctuationInName.

Follow-up

The display-text path uses a different escape contract (FileMaker's own quoting convention rather than XML escaping) and is tracked separately in #221 — script/layout/menu-set names containing " produce ambiguous display text. Round-trip probably still works via greedy regex but isn't covered by tests; lower priority than this fix.

[github-actions]

Test Results

✔️ Tests 1427 / 1427 - passed in 19.2s
✔️ Coverage 79.42% - passed with 70% threshold
📏 15266 / 17688 lines covered 🌿 5192 / 8072 branches covered
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✏️ updated for commit d9fcaf7