v2: canonicalize — namespace prefix re-declaration on child elements

[MeatPopSci1972]

Context

Discovered during Issue #51 follow-up review. Filed as a known limitation of the v1 canonicalizer.

Problem

When an SVG uses namespace-prefixed attributes on child elements (e.g. xlink:href on <use>), @xmldom/xmldom may re-declare namespace prefixes on child elements during serialization. This can produce non-canonical output for SVGs that use xmlns:xlink or other namespace prefixes beyond the base xmlns.

Current behavior (locked in by test)

The xlink:href probe in the existing test suite shows that xmlns:xlink currently appears exactly once (on root) with no child re-declaration — this is the stable case. However, more complex namespace usage (multiple prefixed attributes, deeply nested elements, mixed namespace scopes) has not been tested and may produce redundant xmlns: declarations.

Why deferred

• SequenceForge's own SVG output does not use xlink: or other namespace prefixes — this is a future-consumer concern
• Fixing this requires either a post-serialization namespace deduplication pass or replacing XMLSerializer with a custom serializer
• The v1 canonicalizer is correct for all current SequenceForge SVG output

Gate condition for this issue

The existing known-limitation test in sequence-builder.canonicalize.test.js must be updated (or extended) when this is fixed. The test name is:

Known limitation: xlink namespace on use element canonicalizes without redundant re-declaration

Out of scope here

• d, viewBox, transform, points numeric normalization (separate v2 issue)
• Default attribute elision

References

• Closes follow-up B from Issue Canonical SVG serializer #51 review
• Blocked by: nothing — purely additive when picked up