Emit daily_data module health stamp after daily_closes.collect

[cipher813]

Summary

Adds `health/daily_data.json` emission after `daily_closes.collect()` in `_run_daily()`, using the schema consumed by the executor's `check_upstream_health()` (`alpha-engine/executor/health_status.py:91`).

Belt-and-suspenders with the direct `LastModified` gates in alpha-engine#53 and alpha-engine-predictor#33:

Check	Catches	Catches that
Direct `LastModified` (already in #53/#33)	"stamp green, blob stale"	—
Module health stamp (this PR)	"ran and failed" with structured reason	partial-success `degraded` state, observability, post-mortem metadata

The stamp alone would leave the silent-upstream-staleness hole — stamp says green while the blob is yesterday's. That's why both PRs exist.

Change

• New `_write_module_health()` helper matching executor's `write_health()` schema (`module`, `status`, `last_success`, `run_date`, `duration_seconds`, `summary`, `warnings`, `error`).
• Called in `_run_daily()` immediately after `daily_closes.collect()`, scoped to that collector alone (feature store + ArcticDB append have their own lifecycle; the executor reads `daily_closes/{date}.parquet`, so the stamp reflects only that).
• Emitted on both success AND failure — `last_success: null` when status=failed lets readers distinguish "ran and failed today" from "hasn't run in N hours."
• Legacy `_write_health_marker()` phase stamps (`health/data_phase0.json` etc.) kept untouched for backward compat.

Test plan

• 5 new unit tests in `tests/test_module_health.py` — ok/failed/degraded/key pattern/defaults
• Full suite: 71 passed / 0 failed
• Post-merge: first weekday run (Mon 13:05 UTC) emits `health/daily_data.json` with status=ok, tickers_captured≈909

Deploy sequencing

1. Merge this PR first — data stamps start emitting on next weekday run
2. Then merge follow-up alpha-engine PR that adds `"daily_data": 24` to `_UPSTREAM_MAX_AGE_H`
3. If executor PR merges first, every executor run fails with "no health data returned" until data PR deploys

Follow-up (not in this PR)

• Migrate Phase 1 and Phase 2 stamp writes from `_write_health_marker(phase=N)` to `_write_module_health(module_name=...)`. Out of scope for belt-and-suspenders on `daily_data`.
• Partial-success semantics (status=degraded when N < threshold tickers captured). Placeholder in schema; policy decision for a later PR.

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