[repo-assist] perf: hoist regex literals in C# ERROR-node heuristics to module scope

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What

The C# preprocessor ERROR-node/malformed-declaration complexity heuristics (getComplexityFromErrorNode, getComplexityFromMalformedDeclaration, and their *Reason counterparts in csharpAnalyzer.ts) used inline regex literals that get recompiled on every call. These heuristics run whenever tree-sitter cannot parse a method body cleanly due to preprocessor directives (#if/#else), which can happen multiple times per file in preprocessor-heavy C# code.

This PR hoists all of those regex literals to module-level const declarations (e.g. IF_KEYWORD_REGEX, LOGICAL_OPERATOR_GLOBAL_REGEX, TERNARY_OPERATOR_REGEX), avoiding repeated regex compilation on each invocation - the same "hoist to module scope" pattern already used elsewhere in this codebase (see codeLensProvider.ts's excludeRegexCache).

Why this is safe

• Identical patterns, same test order and short-circuiting logic - purely a compile-once vs compile-per-call change.
• No output/behavior change, confirmed by the full unit test suite (including the dedicated CSharp preprocessor ERROR node reason branches and malformed declaration fallback tests) passing unchanged.

Test Status

• npm run compile: clean
• npm run lint: clean
• npm run test:unit (mocha + c8 coverage): 225 passing, coverage unchanged (83.83% stmts / 93.66% branch / 96.19% funcs / 83.83% lines vs baseline 83.84% - negligible, no regression; the tiny stmt-count shift is purely due to added const declarations, not reduced coverage)
• npm test (full vscode-test suite): not run - blocked by sandbox network restrictions (VS Code download), as documented in repo instructions; expected in this environment.

Proactive Task 5 (Coding Improvements) PR, complementing the companion perf PR for node.children allocation avoidance from the same run.

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