[Repo Assist] Add CompilerMessage warning to groupBy/groupByAsync (closes #125)

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Implements dsyme's request to add a [(CompilerMessage)] warning to groupBy and groupByAsync.

What changed

Both groupBy and groupByAsync now emit an FS9999 compiler warning at every call site:

The result of groupBy must be consumed with a parallel combinator such as AsyncSeq.mapAsyncParallel. Sequential consumption will deadlock because sub-sequence completion depends on other sub-sequences being consumed concurrently.

The warning is added to both the .fs (implementation) and .fsi (signature) files so it fires regardless of how the module is referenced.

To suppress the warning in code that correctly consumes the result in parallel, callers can use #nowarn "9999" locally.

Root cause

groupBy/groupByAsync create sub-sequences whose completion is interdependent — the outer sequence must advance to let each sub-sequence complete. Sequential consumption (e.g. with mapAsync) blocks waiting for a sub-sequence to finish while the sub-sequence waits for the outer sequence to advance, causing a deadlock. Previously this was only documented in the XML doc comment, which many users miss.

Test Status

✅ Build succeeded
✅ 189/190 tests pass — 1 pre-existing flaky timing failure (bufferByTimeAndCount) unrelated to this change
⚠️ Fable tests: 9 pre-existing build errors in the Fable test project (from v4 migration on main); unrelated to this change

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