fix(node:os): stabilize constants namespace identity

[andrewtdiz]

Summary

• cache node:os constants namespace objects so repeated os.constants and nested constants reads preserve identity
• add the new cached namespace roots to the runtime object-cache GC scanner and root rewrite test
• save the Deno node:os constants reference used for this slice

Verification

• cargo fmt --all --check
• cargo check -p perry-runtime
• cargo test -p perry-runtime test_gc_init_mutable_scanner_families_rewrite_runtime_slots
• ./scripts/check_file_size.sh
• ./scripts/regen_api_docs.sh
• ./run_parity_tests.sh --suite node-suite --module os --filter eol-and-constants-identity
• ./run_parity_tests.sh --suite node-suite --module os --filter constants
• ./run_parity_tests.sh --suite node-suite --module os (34 pass / 5 fail; remaining failures are unrelated existing gaps, with userinfo/encoding-buffer covered by fix(node:os): return buffers for userInfo buffer encoding #1884)