fix: stop reporting reachable C# code as unreachable

[SirCrest]

Problem

detect_unreachable_code reports reachable C# as dead code. On a 211k-LOC
C# codebase it produced 7 unreachable_code findings and all 7 were false:

Unreachable code after yield_statement   Sussudio/Services/Runtime/FfmpegRuntimeLocator.cs:114
Unreachable code after yield_statement   Sussudio/Services/Runtime/FfmpegRuntimeLocator.cs:123
Unreachable code after yield_statement   tools/Common/PresentMon/PresentMonProbe.cs:278
Unreachable code after yield_statement   tools/Common/PresentMon/PresentMonProbe.cs:279
Unreachable code after return_statement  Sussudio/Services/Runtime/FfmpegRuntimeLocator.cs:202
Unreachable code after return_statement  Sussudio/Services/Runtime/NativeFfmpegCapabilityProbe.cs:335
Unreachable code after return_statement  Sussudio/Services/Audio/WasapiAudioCapture.cs:907

Three distinct causes:

1. yield_statement was in _TERMINATOR_TYPES, with a comment conceding it
   is "not strictly terminating, but often last in generators". Consecutive
   yield return statements in an iterator are all reachable — each resumes when
   the caller pulls the next element — so any iterator with more than one yield
   in a block got flagged.

2. C# local functions are hoisted. return result; followed by
   void Check(int code, string op) { ... } is idiomatic C#, but the declaration
   after the return was reported as unreachable.

3. Nodes inside a tree-sitter ERROR region were reported. Where the parser
   loses sync the tree no longer describes real control flow; the
   WasapiAudioCapture.cs:907 hit above is an ERROR node whose text is just }.

Fix

• Drop yield_statement from _TERMINATOR_TYPES. It is not a terminator in any
  supported language (Python yields parse as expression_statement, JS as
  yield_expression), so this only removes the C# false positives.
• Add _HOISTED_DECLARATION_TYPES = {"local_function_statement"} and skip those
  when scanning past a terminator.
• Skip ERROR nodes when reporting.

After the fix those same four files report [].

Tests

Four regression tests added to TestUnreachableCode:

• test_csharp_local_function_after_return_is_reachable
• test_csharp_consecutive_yields_are_reachable
• test_error_nodes_are_not_reported_as_unreachable
• test_csharp_statement_after_return_is_still_flagged — pins that genuine dead
  code after a return is still reported, so this does not weaken the detector.

python -m pytest desloppify/tests/lang/ -q → 603 passed. The two failures in
test_treesitter_imports_direct.py and the 34 in the full suite are present on
an unmodified checkout of main and are unrelated to this change.

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