Fix #836: iterator lowering supports try/finally around yield

[DavidObando]

Summary

Iterator (yield) bodies now bind and lower correctly with try { … } finally { … } around yield, and the C#-forbidden try { yield … } catch (…) { … } combination is rejected with a clean diagnostic (GS0367). Closes the #806 workaround in the Gsharp.Extensions.Sequences stdlib.

Root cause

The end-to-end iterator lowering for try/finally was already implemented in IteratorMoveNextBodyBuilder + IteratorTryDispatchPlanner (landed under #419). What was missing was the binder-level rejection of try/catch around yield — the previous behaviour silently dropped the catch clause during lowering, which violates ECMA-335 §I.12 / C# §15.14 (yield cannot resume into a protected region that doubles as a CLR exception-handler frame).

The Gsharp.Extensions.Sequences stdlib then had to flatten WindowedIterator / IndexedIterator (the C# baseline shape used enumerator + try/finally + Dispose) when ported during #806, because there was no positive signal that try/finally + yield actually worked.

Fix

Binder (StatementBinder.BindTryStatement):

• New diagnostic GS0367 raised when a yield lexically inside a try block coincides with one or more catch clauses on the same try. Fires once per offending yield.
• The walker descends only the try block, skipping catches/finallies and FunctionLiteralExpression bodies — yields inside nested lambdas do not trip the rejection.

SDK (Gsharp.Extensions.Sequences.SequenceExtensions):

• WindowedIterator[T] and IndexedIterator[T] restored to the explicit GetEnumerator + try/finally + Dispose shape that matches the C# baseline. InterleaveIterator's using let comment refreshed.

Tests

• Binder (7 tests, Issue836IteratorTryFinallyBinderTests): try/finally accepted; try/catch rejected; try/catch/finally rejected; nested try/finally accepted; inner-catch-with-yield rejected; non-iterator try/catch unaffected; nested-lambda yield inside outer try-with-only-finally accepted.
• Emit + IL-verify (4 tests, Issue836IteratorTryFinallyEmitTests): full enumeration runs the finally exactly once; early break runs the finally on Dispose; nested early-break runs inner-then-outer finally; finally runs exactly once on normal completion. All pass dotnet-ilverify (net10.0).
• Interpreter (3 tests, Issue836IteratorTryFinallyInterpreterTests): tree-walking interpreter parity for try/finally + yield, including nested.
• Existing IteratorTryFinallyEmitTests.Iterator_YieldInsideTryCatchFinally_* repurposed to assert GS0367 fires (previously asserted the catch was silently dropped).

Full dotnet test GSharp.sln suite green (5345 tests). Website (npm run build) builds cleanly with no broken links. ilverify clean on every emit fixture.

Follow-ups

None deferred — the implemented scope covers the issue requirements:

• try/finally around yield (full + nested) lowers to verifiable IL.
• try/catch around yield is rejected by the binder.
• The Gsharp.Extensions.Sequences stdlib uses the restored shape.

Refs

Closes #836
Related #806, ADR-0084 §L5, parent #706