Dart 3 idioms and coding standards

Modern Dart 3.x, value-type-first, total functions for domain logic. A language feature earns its place when it converts a runtime silence into a compile error; everything else is decoration. This skill governs how each declaration is typed, named, kept immutable, and kept total — the error-handling architecture that rides on top of these mechanics lives in error-handling-typed-results.

Read the reference for the task at hand:

• references/construct-verdict-table.md — the feature-by-feature verdict table (sealed / enum / record / class modifiers / extension type / codegen packages), with the rationale for each Use / Skip.
• references/immutability-and-equality.md — immutable value types, copyWith, when to hand-write ==/hashCode, and stable-identity vs value-equality.
• references/complexity-and-honesty.md — the length/nesting limits with their evidence, and the late/!/dynamic honesty-dodge bans in full.

Run scripts/check-dart3-idioms.sh before a PR.

Non-negotiable rules