Serialize the Source view as Exclusive XML Canonicalization

[namedgraph]

Problem

The Source view serialized the canonical tree with serialize(…, indent: true()), which is not a canonical form at all:

• SaxonJS's indenter inserts line breaks and spaces between the element children of mixed content — whitespace that renders, e.g. around a figcaption's <br/>s:

  <figcaption>A single fact, split into its three<br/>
     <br/> parts.</figcaption>

• Empty elements stayed self-closed (<br/>), attribute order was arbitrary, namespace declarations unsorted — none of the c14n rules applied.

Fix

Port LinkedDataHub's ldh:canonicalize-xml pattern:

• Vendor LDH's xml-c14n-sync.js bundle (W3C Exclusive XML Canonicalization, xml-exc-c14n#WithComments — the rdf:XMLLiteral value space, i.e. the form this content is headed for in the LDH v6 format) into lib/, loaded alongside SaxonJS3.js by index.html and the test fixtures.
• New local:canonicalize-xml in edit.xsl: serialize() → parse-xml() bridges the XDM temporary tree to the browser DOM the lib walks, then canonicaliseSync produces the canonical string. The view-source handler now emits that instead of the indented serialization.

Output is now e.g. …its three<br></br><br></br> parts. — start/end tag pairs for empty elements, sorted attributes (<img alt="…" src="…">), sorted namespace declarations, no serializer-injected whitespace; only the document's own whitespace survives.

Note: exclusive c14n (not c14n 1.1) is deliberate — it's what rdf:XMLLiteral specifies and what LDH uses for XMLLiteral persistence; for standalone fragments with no inherited namespace context the two coincide.

Embedding contract

Host pages now include lib/xml-c14n-sync.js next to SaxonJS3.js (README + CLAUDE.md updated).

Testing

• All 15 Playwright suites pass (view-source assertions are whitespace-tolerant and unaffected by attribute sorting).
• Headless extractor/canonical/lint suites pass.
• Browser probe confirms the c14n form end-to-end on the fixture page.

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