Dart target: port LINQ runtime (Enumerable, Grouping) to metano_runtime

[danfma]

Context

The TypeScript runtime (`targets/js/metano-runtime/src/system/linq/`) ships 24 lazy LINQ enumerables (`select`, `where`, `group-by`, `order-by`, `take`/`skip`/`zip`/`union`/etc) plus a `Grouping<K, T>` helper. The Dart runtime (`targets/dart/metano_runtime/`) does not have an equivalent yet — Dart's built-in `Iterable` methods cover only part of the surface, and the parts that diverge (the C# `OrderBy`/`ThenBy` chain, `GroupBy` returning `IGrouping`, the lazy-zip/union semantics) need a real port.

Scope

• Inventory which LINQ methods can lean on Dart's built-ins (`Iterable.where`/`map`/`fold`/`take`/`skip`) and which need a port.
• Port the genuinely missing helpers as Dart classes / extension methods on `Iterable`. Match the TS surface so generated code stays portable.
• Add a `Grouping<K, T>` Dart class implementing `Iterable` and exposing `Key`, mirroring the TS counterpart used by `GroupBy`.
• Update `IrRuntimeRequirementToDartImport.Map` to surface `Enumerable` / `Grouping` as `metano_runtime` imports.
• Add Dart-side tests in `targets/dart/metano_runtime/test/` covering deferred execution, grouping ordering, chaining (`OrderBy` → `ThenBy`).

Verification

• Existing `HashCode` golden tests stay green.
• New tests cover the behavior the TS port locked in.
• A future Dart sample exercising LINQ + records (mirror of `SampleIssueTracker`) regenerates and `flutter analyze` clean.

Part of #28