The freeform mask was assembled on every getMaskDefinition("freeform") call,
re-spreading params and injecting a wrong SquareIcon placeholder, with a type
cast to suppress the structural mismatch. Root cause: MasksRegistry was keyed
on BuiltinMaskType, so freeform had no entry.
- Re-key MasksRegistry from BuiltinMaskType to MaskType
- Register freeform alongside builtins in registerDefaultMasks, with
PenToolAddIcon as its menu icon
- getMaskDefinition and getMaskDefinitionsForMenu become plain registry
lookups with no branching, no manual spreading, no cast
- Rename RegisteredBuiltinMaskDefinition to RegisteredMaskDefinition and
tighten computeParamUpdate return type to Partial<BaseMaskParams>
Co-authored-by: Cursor <cursoragent@cursor.com>
8b8d65b509