[baseballyama]

Summary

Saving no longer writes an element or attribute whose namespace prefix is unbound. Edits such as duplicateSlide and addSlideImage add r:id / r:embed to new elements; when the template declared xmlns:r on individual elements rather than on the part root, the saved part did not parse and the file could not be loaded again.

Motivation

Reported with a reproduction in #456. Namespaces in XML allows declaring a prefix on any ancestor of the element that uses it, and some producers do exactly that. Elements built with elem(...) carry no declarations of their own, so they relied on the part root declaring their prefix.

Closes #456

Changes

• serializeXml tracks the prefix → URI bindings in scope and, on each element, declares any prefix its name or prefixed attributes use that is unbound or bound to a different URI. Declarations the input already had are written exactly as before, so parsed parts serialize byte-for-byte as before; only hand-built elements that previously produced unbound prefixes change.
• One prefix needed with two URIs on the same element is an internal bug and now throws instead of writing ambiguous XML.
• serializeFragment (used for comparison keys) is unchanged.
• getShapeXmlString now writes the shape as a fragment, keeping its output identical to before; it serialized a subtree as if it were a document root, which with the change above would have added xmlns:a / xmlns:p to the shape's elements.

Testing

• test/issue-456-undeclared-prefix.test.ts: both reproductions from the issue (duplicate slide with per-element xmlns:r, then save + reload and remove; image on a slide whose root omits xmlns:r), plus serializer unit cases (declare only where needed, redeclare a rebound prefix, default namespace, conflicting binding). All fail on main.
• Full suite: pnpm typecheck && pnpm lint && pnpm format:check && pnpm test, plus pnpm --filter @office-kit/pptx-editor test and pnpm --filter @office-kit/pptx-dev test.

pnpm vitest run test/issue-456-undeclared-prefix.test.ts

Breaking changes

None

Checklist

• I have read CLAUDE.md and followed the project's conventions.
• I have added or updated tests for the change.
• I have added or updated documentation where user-visible behavior changed.
• If this is a breaking change, I have added a changeset / CHANGELOG entry and
  flagged it above.
• I have re-read my own diff and removed dead code, debug prints, and stale
  comments.
• If I used an LLM to draft this PR, I have verified each change myself, this
  PR represents real work that warrants a maintainer's review, and I am
  willing to defend each line in review.

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