Filter Rust stdlib/dependency paths from profiling by default

[emeryberger]

Summary

When profiling Rust programs, coz was attributing results to lines in the Rust standard library and third-party dependencies. Their source paths (/rustc/<hash>/..., ~/.rustup/..., ~/.cargo/registry/...) were not recognized as system paths, so they passed through source filtering and appeared in profiling output.

• Extract path filtering functions into libcoz/path_filter.h for reuse and testability
• Add is_rust_path() to identify Rust toolchain/dependency paths:
  • /rustc/... — always filtered (synthetic paths embedded by rustc)
  • $HOME/.rustup/... — filtered only when under the user's actual $HOME
  • $HOME/.cargo/registry/... and $HOME/.cargo/git/... — same
• Rust paths are filtered by default (with the default % source scope), but an explicit --source-scope overrides the filter
• Remove #[cfg(target_os = "linux")] gate from the Rust crate's coz_get_counter function — dlsym is POSIX-standard, so coz::progress!() and other macros now work on macOS too
• Add unit tests (65 assertions) covering path detection, $HOME scoping, scope override behavior
• Add Rust integration test that builds a stdlib-heavy binary, confirms DWARF debug info contains /rustc/ paths, and validates they are excluded from profiling output

Supersedes #272.

Test plan

• Unit tests pass (65/65 assertions in tests/path_filter/path_filter_test.cpp)
• Rust integration test passes (tests/run_rust_filter_test.sh)
  • Phase 1: Confirms 209 unique /rustc/ paths in DWARF debug info
  • Phase 2: Validates profiling output excludes Rust stdlib paths
• CI build and existing tests pass
• Verify --source-scope '/rustc/%' overrides the filter when explicitly requested

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