Add C++ overload conversion ranking

[azizur100389]

Summary

• add a C++ primitive overload conversion ranker for exact, promotion, and standard conversion ranks
• thread the ranker through overload narrowing for C++ resolver call paths
• add unit coverage and C++ resolver fixtures for int/double overload selection

Fixes #1578

Tests

• npx vitest run test/unit/scope-resolution/pick-implicit-this-overload.test.ts test/unit/scope-resolution/overload-narrowing.test.ts
• npx vitest run test/integration/resolvers/cpp.test.ts -t "C\+\+ overload"
• npx vitest run test/integration/resolvers/cpp.test.ts
• npx tsc --noEmit
• git diff --check

Note: full parallel npm test still shows existing CLI e2e stdout/EPIPE failures from shared test state; test/integration/cli-e2e.test.ts passes in isolation, and test/integration/mcp/server-startup.test.ts passes in isolation. A sequential full-suite run exceeded the local 30-minute timeout.

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[azizur100389]

Closing as superseded by upstream PR #1606, which has already merged and closed #1578. This branch now conflicts with the merged implementation, so keeping it open would duplicate the same overload-ranking work.