feat: support bound arrows via skeleton API start/end format

[sammcj]

This PR fixes issues where agents using the MCP server would create diagrams but not correctly connect / anchor arrows and lines to objects, leading to them floating where if you move an object the line or arrow would not remain connected. #33

Summary

• Document and implement the skeleton API's start/end arrow binding format, which lets agents create connected arrows without computing coordinates manually
• Add resolveArrowPositions post-processing step that computes arrow geometry from bound shape edges, fixing the gap where convertToExcalidrawElements creates binding metadata but leaves arrows unpositioned
• Update cheat sheet to teach bound arrows as the preferred format over manual x/y/points
• Fix null-coordinate guard to use != null instead of isNaN check, avoiding false positives when arrow coordinates are legitimately 0

Problem

AI agents consistently produced diagrams with floating arrows because:

1. The cheat sheet documented startBinding/endBinding (inert output metadata) instead of the skeleton API's start/end input format
2. convertToExcalidrawElements sets up bidirectional binding metadata but does not compute arrow x/y/points — arrows are left at undefined coordinates
3. exportToSvg (used for all rendering) cannot render arrows without coordinates

Changes

src/server.ts — Cheat sheet updates:

• Add bound arrow as preferred format with start/end by target element ID
• Replace startBinding/endBinding docs with skeleton start/end format
• Update drawing order guidance for bound arrow element ordering
• Simplify two-box example to use bound arrow

src/mcp-app.tsx — Arrow position resolution:

• Add resolveArrowPositions(): computes arrow x/y/points from bound shape edge midpoints using closest-cardinal-point geometry
• Also resolves null-coordinate labels on bound arrows
• Add contextElements parameter to convertRawElements so arrows can resolve binding targets from checkpoint base elements
• Pass checkpoint base as context in both streaming and final render paths

CLAUDE.md — Correct project documentation to reflect skeleton API usage

[vercel]

@sammcj is attempting to deploy a commit to the Excalidraw Team on Vercel.

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[sammcj]

@excalibot @excalidraw

[sammcj]

Sorry to ping you directly but @dwelle any chance this could be considered for merging as it significantly improves the quality of the output generated with this MCP server.

[aminezar]

Was this implemented?

[sammcj]

Nope, no response from the maintainers

[sammcj]

@antonpk1 maybe?