feat: add support for custom vector shape

[VladaHarbour]

Before:

After:

[linear]

SD-1499 Bug: Signature underline missing + “By:” alignment shifted after DOCX import

[github-actions]

Visual diffs detected

Pixel differences were found in visual tests. This is not blocking — reproduce locally with cd tests/visual && pnpm docs:download && pnpm test to review diffs.

[github-actions]

⚠️ AI Risk Review — potential issues found

• SVG path commands from DOCX are not validated - only structural checks on the geometry object, allowing potentially malicious or malformed SVG commands
• extractCustomGeometry() uses scaleX/scaleY but doesn't validate that intrinsicWPx/intrinsicHPx are positive before division, could divide by zero if EMU values are 0
• deriveBlockVersion concatenates path strings without length limits, very complex paths could cause performance issues in cache key comparison

_{Via L3 deep analysis · sensitive risk}

[caio-pizzol]

@VladaHarbour thanks for the work on this! custom geometry support (a:custGeom) has since been implemented on main via #2105 (SD-1877) — extraction, rendering, and tests are all in place. closing as superseded.