Great contribution — the implementation is clean, follows all project conventions, tests are thorough, and the use case doc is well structured. One small fix needed before merge, plus two observations for your awareness.

Fix needed

index.html — null checks for optional fields

parseProjectJson signals "field not present" by returning null for gridSize and scaleFactor. The import handler currently checks truthiness, which is inconsistent with that contract:

// current — falsy for 0 (not a real bug, but imprecise)
if (parsed.gridSize) { ... }
if (parsed.scaleFactor) { ... }

// preferred — matches what parseProjectJson returns for "missing"
if (parsed.gridSize !== null) { ... }
if (parsed.scaleFactor !== null) { ... }

Observations (no action needed)

• No range validation on imported values — a corrupted file could set gridSize: 9999 or scaleFactor: -5. Low risk since these are the user's own files, but something to consider if the format ever becomes shareable.
• version field is written but never read — fine for now, but worth a note: if a v2 format is added later, old code will silently apply it. A version check in parseProjectJson would future-proof this.

Fix the null checks and this is good to merge. Thanks!