Shard-manifest schema mismatch: #1902 upload_text_payload writes {shards:[...]}, shared hub.stage_sharded_text only reads {parts:[...]}

Goal

Make every sharded text artifact written by upload_text_payload readable by the shared hub.stage_sharded_text, so a consumer on a fresh pod or a sibling issue can restage #1902's rollout files (and any future issue's) through the common path instead of hand-rolling a reader.

What is wrong

Two producers in the repo write a <stem>.manifest.json with different schemas, and the shared stager understands only one of them.

Producer A, the #2054 shared path (_shard_large_jsonl_for_upload):

{"parts": ["<name>", ...], "sha256": {"<name>": "<hex>"}}

Producer B, scripts/issue1902_run.py:571 (upload_text_payload):

{"source": "B_k5_seed45.jsonl", "n_shards": 2,
 "shards": [{"name": "...", "n_lines": 13573, "sha256": "..."}]}

hub._parse_shard_manifest (src/explore_persona_space/orchestrate/hub.py:3140-3153) reads man.get("parts") and raises RuntimeError: shard manifest lists no parts when it is absent. So hub.stage_sharded_text fails loud on every file producer B uploaded.

Reproduction (2026-09-09, pod-1902-k5grid):

RuntimeError: shard manifest lists no parts: superkaiba1/explore-persona-space-data:
  issue1902_stage_map/raw_completions/gen/single/B_k5_seed45.manifest.json

Blast radius today: all 16 K=5 rollout files plus the original seed-42 rollout files under issue1902_stage_map/raw_completions/gen/. The failure is fail-loud, so nothing silently corrupts. scripts/issue1902_k5_fits.py:228-245 already carries a private reader for producer B's schema, which is the duplication this task removes.

Fix

Teach _parse_shard_manifest both schemas: keep parts + sha256, and accept shards: [{name, sha256, n_lines}] by projecting it onto the same (parts, sha256_by_name) return. Prefer the richer form's n_lines as an extra post-concat check where present. Then delete the private reader in issue1902_k5_fits.py and route it through hub.stage_sharded_text.

Do NOT change producer B's on-Hub schema: files already uploaded under it must stay readable, so the reader is the place to converge.

Acceptance:

• hub.stage_sharded_text restages issue1902_stage_map/raw_completions/gen/single/B_k5_seed45.jsonl to 16,391 rows with every shard sha256-verified.
• A unit test per schema, plus one asserting a manifest with neither key still raises.
• issue1902_k5_fits.py no longer parses manifests itself.

Provenance

Found while staging capture inputs for the #1902 full-grid K=5 round on a fresh pod (that round worked around it with a throwaway pod-side reader mirroring issue1902_k5_fits._rollout_records; the workaround is not committed). Not auto-dispatched: this VM has automated Claude sessions disabled, so the task is filed at proposed for manual pickup.