Ship generic-attribute syntax once Unity bundles Roslyn 4.4+

[svermeulen]

The source generator already reads AttributeClass.TypeArguments (committed in b7ada7a5d81), so the moment the Trecs core package can declare SingleEntityAttribute<T1..T4> / ForEachEntityAttribute<T1..T4> / FromWorldAttribute<T1..T4>, users get the C# 11 generic-attribute shorthand:

[SingleEntity<TuskTags.Player>]            // vs. [SingleEntity(typeof(TuskTags.Player))]
[ForEachEntity<TuskTags.Fish, Eating>]     // vs. [ForEachEntity(typeof(Fish), typeof(Eating))]
[FromWorld<TuskTags.Fish>]

Why it isn't shipped today

Unity 6's bundled Roslyn predates 4.4, so generic attributes are stuck behind -langversion:preview. We tested both paths:

• -langversion:11 — rejected outright: CS1617 Invalid option '11' for /langversion.
• -langversion:preview — accepts generic attributes, but simultaneously activates stricter C# 11 ref-safety analysis. That surfaces ~10 pre-existing CS8347 / CS8168 errors in NativeWorldAccessor.cs and EntitySubmitter.cs (Cannot return local 'bag' by reference, may expose variables referenced by parameter), which is a real refactor outside the scope of this feature.

What's needed when we revisit

1. Confirm Unity ships Roslyn 4.4+ (or fix the ref-safety regressions to opt into preview cleanly).
2. Add csc.rsp with -langversion:latest (or :11) at Packages/com.trecs.core/Scripts/.
3. Re-add the four-arity generic attribute classes to SingleEntityAttribute.cs, ForEachEntityAttribute.cs, FromWorldAttribute.cs — capped at 4 to match WithTags<T1, T2, T3, T4>. ForEachEntity's generic variants need Set / MatchByComponents properties; the others are bare.
4. No source-gen changes required — TypeArguments parsing is already wired through InlineTagsParser, IterationCriteriaParser, the FromWorld paths in JobGenerator/AutoJobGenerator, and AutoSystemGenerator.HasAnyAttributeCriteria. The conflict diagnostic (TRECS053) already covers [X<T>] mixed with named Tag / Tags.
5. Tests are already in place — RunOnceGeneratorTests.RunOnce_GenericAttribute* exercise the path via stubs.