refactor: remove dead tangent handle variant

The { kind: "tangent" } MaskHandleId variant was never constructed — neither
getFreeformDisplayHandles nor any other code ever produced a tangent handle
object. The in/out control-arm branches in computeFreeformParamUpdate, the
tangent case in maskHandleIdKey, and the CUSTOM_MASK_TANGENT_SIZE rendering
path in mask-handles.tsx were all unreachable. Removed all four dead paths
and removed "tangent" from MaskHandleKind.

Co-authored-by: Cursor <cursoragent@cursor.com>