[codex] Harden pf anchor references

[madeye]

Summary

• fail macOS pf setup if /etc/pf.conf cannot be read instead of loading an empty main ruleset
• build the pf main ruleset through a tested helper that preserves existing content
• detect rdr-anchor and filter anchor references as distinct directives so both are loaded

Tests

• cargo fmt --all -- --check
• cargo test firewall::pf::tests
• cargo test
• cargo clippy --all-targets -- -D warnings
• git diff --check