Show the remaining shared examples in both backend languages

[woksin]

Stacked on docs/arc-multi-implementation (#2708). Review that first; this branch contains only the commits below.

Thirteen shared examples still showed C# only.

Changed

• The page on using current state in a command shows both languages. It had been set aside on the belief that the JVM has no way to say which thing a command acts on — that came from reading its documentation rather than its source. @CommandKey and CommandKeyProvider have been there all along.
• The six frontend pages that carried a single backend illustration now show both. They sit on framework-neutral pages and both backends generate the same client, so a JVM reader should see their own backend.
• The frontend overview loses its full-stack tab wrapper. It was hardwired to a slot labelled C# — backend, so a Kotlin tab would have sat inside a tab called C#. The TypeScript is a consequence of either backend rather than an alternative to one, so it reads better flat.
• Where the two genuinely differ the prose says so rather than implying symmetry: C# resolves a read model into a validator's constructor while the JVM resolves it for the handler; C# also accepts an EventSourceId<T>-typed property where the JVM reads the command key; and the two client-projected validation subsets are not the same.

Fixed

• The Chronicle scenario examples could not be compiled because the Chronicle testing project was not referenced by the snippet validator. They were unchecked strings; they are now compiled like the rest. All 33 C# snippets compile against real Arc source.

Not included

The tutorial chapters. They are buildable scaffolds against two pinned toolchains, and nine more snippets plus per-backend setup prose is the highest-stakes onboarding path to change in the same breath. Worth its own change.