feat(dart): add external-type serialization

[chaokunyang]

Why?

Dart classes owned by dependencies cannot carry Fory annotations. Applications
need generated serializers for those types without copying values into local
models or adding a second registration, carrier, root, or polymorphic path.

What does this PR do?

• Adds ForyStruct.target and constructor selection so an abstract final
  local schema declaration generates a direct serializer for a closed external
  class.
• Uses the existing generated struct model, target-keyed schema/module
  registration, field descriptors, carrier codecs, root APIs, reference
  resolver, compatible reads, and polymorphic registration.
• Generates direct target getters, generative constructor calls, and setters;
  validates the external contract at generation time, including generic type
  rendering, re-exported nullable aliases, and constructor-cycle constraints.
• Covers roots, fields, nested list/set/map fields, named and numeric
  registration, polymorphism, references, compatible mode, generic targets,
  Java-to-Dart xlang exchange, and JavaScript compilation.
• Documents the current external-type serialization and manual serializer
  APIs without migration or removed-API material.

Related issues

N/A

AI Contribution Checklist

AI Usage Disclosure

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

The public Dart annotation API adds optional target and constructor
arguments to ForyStruct. The wire format is unchanged.

Verification

• cd dart/packages/fory && dart run build_runner build && dart analyze && dart test
  • generation: 98 outputs, no tracked changes
  • analysis: clean
  • tests: 232/232 passed
• cd dart/packages/fory-test && dart run build_runner build && dart analyze && dart test
  • generation: 3 outputs, no tracked changes
  • analysis: clean
  • tests: 18/18 passed
• cd dart && dart analyze && dart test
  • analysis: clean
  • tests: 1/1 passed
• cd dart/packages/external-type-test-models && dart analyze
  • analysis: clean
• dart compile js for
  dart/packages/fory-test/tool/external_type_js_smoke.dart, followed by Node
  execution
  • compiled successfully and exited successfully
• FORY_DART_JAVA_CI=1 ENABLE_FORY_DEBUG_OUTPUT=1 mvn -T16 --no-transfer-progress test -Dtest=org.apache.fory.xlang.DartXlangTest
  • 101/101 passed with no failures, errors, or skips

Benchmark

Fresh apache/main ordinary benchmarks were run in adjacent baseline/current
pairs. Every retained median met the less-than-1% slowdown gate, and the
generated ordinary benchmark source was byte-identical.

Equivalent ordinary/external generated paths were measured in AOT mode with
split debugging data:

Path	Retained delta
Root serialize	+0.02%
Root deserialize	+0.04%
Direct field serialize	+0.53%
Direct field deserialize	-0.82%
List field serialize	+1.47%
List field deserialize	-0.02%
Map field serialize	+0.09%
Map field deserialize	+0.90%

Negative values are slowdowns; every slowdown is below 1%. Serialized bytes
and normalized generated serializer bodies matched. The fixed-iteration VM
allocation check reported identical per-class object counts for all eight
serialize/deserialize lanes at 1,000 iterations.