Make wastewater and hospital sample fallbacks fail-soft on aged samples

[Hefrock]

Summary

Follow-up to the weather fix (#17): the same aged-sample failure mode lives in the wastewater and hospital sample tiers, which clip to [start, end] the same way. They aren't failing CI yet only because their fallback tests use wide windows that still overlap the frozen sample — a latent time-bomb that fires once the committed sample ages ~365 days further and a today-relative window no longer overlaps it, silently dropping the signal to zero rows.

• Extract the whole-year shift into src/ingestion/sample_window.py::shift_sample_to_window so all three fetchers share one tested definition; route weather through it too (removing its inline copy).
• Gate the shift to the synthetic sample tier only, never real data:
  • hospital._load_csv gains a shift_to_window flag — True for the sample, False/default for the real MA DPH cache (Tier 1).
  • wastewater shifts in _load_sample before the shared _localize_and_window, so the real WastewaterSCAN/MWRA/CDC parsers that also use that helper keep clipping out-of-window data.
• The planted signal is day-of-year seasonal, so a whole-year shift preserves its seasonal position; shifting naive timestamps before localizing keeps each caller's DST guards in charge.

Test plan

• pytest tests/ -q — 112 passed (+8 new)
• New aged-window regression tests for weather, wastewater, and hospital sample tiers (window years past the frozen sample → still non-empty and in-range)
• Guard test that real MA DPH data (shift_to_window=False) is still clipped, never shifted
• Helper unit tests: no-op when overlapping, shift forward/back, empty-frame safe, leap-year month/day preservation
• End-to-end: ran python -m src.ingestion.run --city boston (default trailing-365 window, as the daily cron does) with network restricted so every fetcher hits its sample tier — all signals produced non-empty output, accumulated weather.parquet had 0 duplicate timestamps, no crashes

https://claude.ai/code/session_011FPpSio8V9rxWMCG5RnYxB

---

Generated by Claude Code