docs: rename manual serializers to custom serializers

[chaokunyang]

Summary

• standardize the repository on the canonical term custom serializer
• destructively rename six guide, example, test, and fixture files and update every reference
• align runtime comments, diagnostics, tests, benchmarks, specifications, READMEs, and agent guidance without keeping aliases or redirects

Why

Custom serializer describes an application-defined serialization implementation more precisely than manual serializer and is already the established term in several language guides. Keeping both terms created inconsistent documentation, filenames, test names, and diagnostics for the same concept.

Verification

• stale terminology, path, ID, identifier, and filename scans: zero matches
• relative links in 50 changed Markdown files: all targets exist
• C#: format check and 312 tests
• Dart: code generation, analysis, and 334 + 32 tests
• Rust: full test suite and Clippy (allowing only the pre-existing constant-assertion lint)
• Swift: format, lint, and 229 tests
• Python: Ruff and 690 tests
• Java: Spotless and compile with JDK 21
• JavaScript: Prettier and ESLint
• Go: gofmt and full tests
• C++: clang-format check
• Rust and Swift external-type benchmarks: compile checks