verify_plan: flag N>1 concurrent producers writing one phase_outputs dest with no per-producer token or merge step (output-side sibling of #2236/#2237)

Goal

Add a verify_plan.py check (WARN-first is acceptable): a plan §9 phase that declares N>1 CONCURRENT PRODUCERS of one output family (e.g. "6 pods ... sharded {behavior} x {seed-half}") must have phase_outputs dest templates carrying a per-producer distinguishing token (<half> / <seed> / <suffix> / <shard>), OR a named merge/union step consuming every shard before any coverage assert. Absent both, the second producer's upload_folder silently clobbers the first at identical repo-relative paths, each producer's own presence-based upload-verify PASSes, and verify-then-terminate destroys the only surviving copy before the fold's coverage assert can catch the loss.

Provenance

workflow_fix_target: scripts/verify_plan.py Surfaced by BOTH methodology reviewers (Claude + Codex twin) on task #1739's amendment plan v25 (round composition-grid-multiseed-plus-arm2-repair, 2026-08-22): the plan sharded leg 1 as 6 pods = {evil, syco, hall} x {seeds 0-2, seeds 3-4} while §4 pinned ONE out-root per behavior and the rig writes fixed relative paths (arm_results/all_arms_spearman.json, arm_results/percell/cells.jsonl — issue1739_fits.py:416/:1716); phase_outputs.L1-compose uploaded both halves to the same HF prefix with no per-half component and no named union step. Caught at plan review; would otherwise have destroyed half the 370-cell grid post-terminate.

Design sketch

This is the OUTPUT-side sibling of the existing input-side checks c58 (#2237 fan-out pod-name) and the #2236 fan-out-staging extension. Detection sketch: in §9 / parallelism prose, detect a concurrency declaration (N pods/workers x an axis) tied to a phase; for that phase's phase_outputs entry, require either (a) a template token from a small vocabulary (<half>, <seed>, <shard>, <suffix>, <worker>, seed<S>, per-producer subdir) in the outputs/dest strings, or (b) a merge/union vocabulary hit ("union", "merge", "concatenate the shards") in the same phase block or a downstream phase that names the producing phase. Calibrate against the persisted plan corpus before promoting WARN->FAIL (the c12 calibration precedent).

Acceptance criteria

1. The #1739 v25 shape (N>1 producers, shared dest, no merge step) triggers the check; the v26+ corrected shape (per-half dest template + fold union step) passes.
2. Corpus calibration run recorded (flip table) — no more than a handful of false flips on historical plans, adjudicated in the task body.
3. Check registered in the verify_plan checks list + LESSONS/docs surface per the repo convention for new checks; pin test added.