Document source-file and declaration-shape diagnostics

Three diagnostics are acknowledged in the catch-all article but still need durable homes and article-home decisions: `CS9343` (can't pass base-type arguments without a parameter list on the type declaration), `CS9348` (a compilation unit can't directly contain fields/methods/properties), and `CS9378` (`#!` must be the first characters on the first line).

Give each an explicit owner: `CS9343` fits `constructor-errors.md` or a primary-constructor-adjacent article, and should show the declaration shape that permits base arguments, not just the error text. `CS9348` needs an appropriate syntax/declaration-placement home (this decision is still open). `CS9378` fits `preprocessor-errors.md` and/or file-based-program guidance, explained in the context of file-based programs and shebang handling. All three should move from catch-all acknowledgment to specific article ownership. See [Roslyn error codes](https://github.com/dotnet/roslyn/blob/main/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs#L2444) for exact text.

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[Associated WorkItem - 632364](https://dev.azure.com/msft-skilling/Content/_workitems/edit/632364)