fix: find core and extended properties through the root relationships (#457)

[baseballyama]

Summary

getCoreProperties / setCoreProperties and getExtendedProperties / setExtendedProperties now find their parts through the package-root relationships instead of the fixed names /docProps/core.xml and /docProps/app.xml. Packages that store core properties elsewhere (for example package/services/metadata/core-properties/<guid>.psmdcp) previously read as having no properties, and setting them added a second core-properties part and relationship.

Motivation

Reported with a reproduction in #457. OPC (ECMA-376 Part 2) locates the core-properties part by the root relationship of type …/metadata/core-properties, and a package may have at most one; the part name is up to the producer.

Closes #457

Changes

• The four helpers resolve the part from the internal root relationship of the matching type (relative or absolute target). A part at /docProps/core.xml that no relationship points at is no longer treated as the core properties.
• When setCoreProperties has to bootstrap, it replaces such an unlinked /docProps/core.xml instead of failing on the name clash, and drops a core-properties relationship whose target part is missing, so the package ends with exactly one relationship and one [Content_Types].xml override (a stale override for a missing part used to be duplicated).
• If an (invalid) package holds several core-properties relationships, the first whose part exists is used.
• A relationship target that escapes the package root (../…) throws, as it already does for every other relationship the library resolves.
• setExtendedProperties still throws when there is no extended-properties part; the message no longer names /docProps/app.xml.
• Internal: resolveTarget accepts PACKAGE_ROOT as the source for package-root relationships.

Testing

• test/issue-457-core-properties-rel.test.ts: the issue's reproduction (read + update a moved part, one relationship, no new /docProps/core.xml, validator clean), an absolute relationship target, moved app.xml, an unlinked /docProps/core.xml, a dangling core-properties relationship (one override afterwards), and several relationships with the first dangling. All fail on main.
• Full suite: pnpm typecheck && pnpm lint && pnpm format:check && pnpm test, plus pnpm --filter @office-kit/pptx-editor test.

pnpm vitest run test/issue-457-core-properties-rel.test.ts

Breaking changes

None. Packages written by this library or the reference desktop app use /docProps/*.xml with a matching relationship and behave as before.

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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