Support C# contextual async identifiers

[stef-k]

Summary

• add a bounded C# compatibility retry for valid contextual async identifiers
• preserve authored byte/line coordinates and fail closed for ambiguous or malformed syntax
• cover exact public measurements, installed-package behavior, documentation, and changelog

Closes #114

Root cause

The pinned and current Tree-sitter C# grammars misparse valid async expression identifiers and named-argument names. Upstream tracking issue tree-sitter/tree-sitter-c-sharp#276 remains open, so no verified provider release can be pinned for this case.

Validation

• python -m unittest discover -s tests -p test_csharp_contextual_keywords.py -v — 1 passed
• python -m unittest discover -s tests -p test_pattern_guard_complexity.py -v — 11 passed
• python -m unittest tests.test_packaging.InstalledPackageTests.test_installed_syntax_pipeline_loads_parser_dependency -v — 1 passed
• python -m unittest discover -s tests — 306 passed
• disposable-wheel code-guard . --ci — exit 0; REVIEW remains visible and non-blocking
• python -m compileall -q src skills research tests — exit 0
• .agent-tools/code-guard.loc-baseline.json absent and not a symlink
• git diff --check origin/main...HEAD — exit 0

Review

Independent read-only review found no material findings at 55c97f4447c40194d7454311ecf8b0dc3e9e55e7 and returned PASS.