[codex] wire node core subset radar

[andrewtdiz]

Linked issues

• Closes part of compat: Node core test subset comparison job #800

Behavior cluster summary

This wires the existing Node core subset radar into CI as an advisory compatibility signal:

• adds .github/workflows/node-core-subset.yml
• sparse-checks out the pinned Node.js test corpus from test-compat/node-core/pinned-version.txt
• builds Perry's release binary and runs scripts/node_core_subset.py
• supports manual focused runs through a space-separated apis input and max_per_api
• writes a Markdown summary table to the Actions step summary
• uploads test-compat/node-core/report.json as an artifact
• updates test-compat/node-core/README.md with the workflow entrypoint

Why these changes are batched

The runner already existed; this PR adds the missing workflow and documentation together so the radar has a durable scheduled/manual entrypoint and users can find how to run focused subsets.

Tests and checks

• parsed .github/workflows/node-core-subset.yml with local YAML loader
• python3 -m py_compile scripts/node_core_subset.py
• scripts/node_core_subset.py --help
• git diff --check
• ./scripts/check_file_size.sh
• cargo fmt --all -- --check

Known limitations

• I did not run the full Node-core subset sweep locally because it requires a release build plus upstream corpus execution, and an unrelated release/http build was already active in the shared workspace.
• The workflow is intentionally advisory and non-required. Threshold/gating policy can be added after several stable reports establish a baseline.

Non-goals

• Changing radar bucket semantics.
• Expanding the Node common shim.
• Fixing runtime/API failures that the radar reports.
• Making the job a required merge or release gate.