Emit a uniform phase field in reproducibility cards so gate checks can resolve phase-to-card mechanically

Goal

Make the phase a reproducibility card belongs to machine-readable, by having every card writer emit a consistent phase field, so gate checks can resolve phase → card without guessing from filenames.

Why

Cards today carry no consistent phase identifier. Measured on #2162 (all at origin/issue-2162):

• eval_results/issue_2162/stage2/stage2_results.json → has phase: "stage2-upload"
• eval_results/issue_2162/margin/upload_done.json → no phase key
• eval_results/issue_2162/gates/pilot_gate_report.json → no phase key, and no token of "grid" / "anchors" / "stage-1" anywhere in its path or content, even though it IS the grid/anchors phase's card

The commit key itself also sits at three different nesting paths within that one issue (reproducibility_card.git_commit, note.reproducibility_card.git_commit, repro.git_commit), plus a top-level final_commit_sha twin.

The concrete consequence: #2191 added a code-sha-cards check to scripts/verify_report.py that verifies a v2 report's Code-SHA rows against these cards. Because no honest label → card resolution exists, its pairing leg (b3) has to approximate with filename-token overlap against a hand-maintained stopword set, and it provably cannot resolve the grid/anchors phase at all — so a report that cites every correct card SHA but pairs them to the WRONG phases is only partially detectable. A uniform phase field would close that at the source and let the pairing leg become mechanical for every future issue.

Scope sketch

1. Decide the field: name (phase), placement (top level vs inside the reproducibility card block), and a controlled vocabulary or free-string convention. Prefer one canonical location so a recursive search is not needed.
2. Emit it from every card writer (the upload_done.json / gate-report / sentinel writers). Additive — do not change existing keys.
3. Document the card shape in the relevant rule so future writers comply.
4. Optionally: teach verify_report.py's code-sha-cards pairing leg to prefer phase when present and fall back to token overlap when absent, so old cards keep working. Retire the stopword set once coverage is good.

Backward compatibility is required: existing cards have no phase, and #2191's check must keep passing on them (its degrade ladder already treats a missing phase as unresolvable-and-skipped).

Provenance

Surfaced as an out-of-scope forward note by the methodology critic during #2191's plan review (2026-08-07), and recorded in #2191 plan v2 §11. #2191 deliberately changes no card writer. Filed as proposed rather than auto-dispatched because the fix lands in card-writing / experiment-infra code rather than the workflow surface, so it wants triage before it consumes a session.