feat: codegen typed_object fields as nullable struct fields

JavaWireFormat typed_object fields that correspond to a known parcelable
are now generated as *ParcelableType struct fields with proper nullable
marshal/unmarshal. Previously these fields were opaque (no struct field,
marshal wrote int32(0) null marker). Now:

- spec2go looks up typed_object field names in a parcelable index built
  from all loaded specs. If a matching parcelable is found, the AIDL
  qualified name is stored in JavaWireField.GoType.

- The codegen generates *Type struct fields from GoType metadata
  (not from FieldDecl, to avoid perturbing the import graph's
  DFS back-edge selection for cycle breaking).

- Marshal writes int32(1) + MarshalParcel for non-nil, int32(0) for nil.
  Unmarshal reads the flag and allocates/unmarshals if non-zero.

- Cross-package imports are handled by the TypeRefResolver. A BFS-based
  WouldCreateCycle check redirects to types sub-packages when importing
  the target would create a cycle (even for edges not in the import graph).

- ImportGraph gains WouldCreateCycle and SameSCC methods for cycle
  detection of edges not captured in the static import graph.

Example: LocationRequest.WorkSource is now *types.WorkSource with proper
nullable serialization instead of a hardcoded null marker.

Author: xaionaro@dx.center