Dart 3 idioms and coding standards

Enforces which Dart 3 construct each declaration earns — sealed class + exhaustive switch with no `default:`/`case _:`, the three class modifiers (`sealed`/`final`/`abstract interface class`) and skip the rest, records as intra-layer tuples only, immutable value types (`final` fields, `const` ctor, value equality) hand-rolled when trivial and `freezed` when boilerplate dominates, explicit stable identity, total non-throwing domain functions, make-illegal-states-unrepresentable, and firm method/build/file/nesting complexity limits (the single-source-of-truth table other skills cite) — while banning `late`/`!`/`dynamic` honesty dodges. Use when authoring or reviewing any Da…