feat(dart): add schema evolution (TypeDef meta share) support

[arjav1528]

Why?

When other languages serialize with schema evolution enabled (TypeDef/meta share), Dart could not decode because it did not implement the TypeDef/TypeMeta read/write flow. This blocked cross-language compatibility for evolving schemas (see #3290).

What does this PR do?

• TypeDef / FieldDef and body parsing: Adds FieldDef and RemoteFieldType (Simple, ListSet, Map) in meta/field_def.dart; extends TypeResolverImpl to parse the full TypeDef body (field header, type info, name bytes or TAG_ID) and to use varuint32 small7 for type IDs in the body and in type meta.
• Meta share cache and context: Read cache stores (TypeInfo, List<FieldDef>) per index; DeserializationContext gets currentRemoteFieldDefs; readTypeInfo sets it when returning from shared type meta (new or ref); resetReadContext clears cache and currentRemoteFieldDefs.
• Compatible-struct read by remote field order: When compatible is true and currentRemoteFieldDefs is set, ClassSerializer uses _readByRemoteFieldOrder: iterates remote field defs, reads values by remote type (including nested LIST/SET/MAP), assigns to local fields by name (snake_case) or by index; supports TAG_ID for future tag-based matching.
• Tests: Round-trip with compatible + ref; meta share cache (single marker for list element type); TypeDef format round-trip; named compatible (typename) and compatible by user type id; cross-language golden test added but skipped with a note for future integration.

Related issues

• Fixes [Dart] add schema evolution (TypeDef meta share) support #3290

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
  No new public APIs; internal types (FieldDef, RemoteFieldType) are in src/meta and used by the resolver/serializer.
• Does this PR introduce any binary protocol compatibility change?
  No. Dart now correctly implements the existing xlang Type Meta / TypeDef format so that Dart can decode data produced by other languages with compatible + meta share enabled.

[arjav1528]

@chaokunyang could you please help me with the PR tests