feat: spec2go — generate struct fields and marshal/unmarshal from JavaWireFormat

Bare AIDL parcelables (like android.location.Location) are declared
without fields in AIDL but have wire format extracted from Java
writeToParcel(). Generate Go struct fields and serialization code
from this wire format data.

Key changes:
- parser.ParcelableDecl gains a JavaWireFormat field for wire layout
- spec2go synthesizes FieldDecl entries from JavaWireFormat when a
  parcelable has no AIDL-defined fields
- codegen generates custom MarshalParcel/UnmarshalParcel that handle
  conditional fields (bitmask guards) and opaque field skipping
- Fields with invalid names, unresolvable conditions, or unknown
  types are treated as opaque and skipped
- Duplicate field names are deduplicated with numeric suffixes
- Java string constants (e.g. LocationProvider) are now generated
  as typed Go constants

Author: xaionaro@dx.center