refactor: collapse mask definition registry to a single keyed registry

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>