fix: support Flutter web compilation

[timcreatedit]

Summary

• use platform-specific integer limits so JavaScript builds avoid unsupported 64-bit literals while native parsing retains its existing limit
• preserve the native FNV-1a hash API and add an exact cross-platform field hash for generated decoders
• add coverage ensuring runtime and generated-code hashes stay aligned

Verification

• dart analyze --fatal-infos --fatal-warnings <changed files>
• dart test in crimson_test (127 tests)
• dart compile js example/main.dart
• flutter build web in a temporary Flutter app consuming Crimson by path (including successful Wasm dry run)

[timcreatedit]

Superseded by the organization-owned fork PR: time-loop#1. ClickUp now consumes the Time Loop repository.