Add WPC Winter Weather Outlook (PWPF) snow/freezing-rain feed

[MikeWills]

Summary

New synthetic-alert feed for WPC's Probabilistic Winter Precipitation Forecast (PWPF), Day 1/2 only. Closes #33.

• Pwpf settings block: Enabled, CheckIntervalSeconds (1800), MinProbabilityPercent (40), SnowThresholds ("1,4,8,12"), IceThresholds ("0.10,0.25")
• WpcPwpfService downloads WPC's per-threshold contour KMZ (/pwpf/latest_kml/prb_24h{snow|icez}_ge{..}_f{024|048}_cntr_latest.kmz), unzips, parses the KML contour rings into NTS polygons, and takes the highest contour level covering any monitored zone/county centroid
• One alert per day per precip type (WPC Day 1 Snowfall Outlook, WPC Day 1 Freezing Rain Outlook, …) when the highest configured threshold's band ≥ MinProbabilityPercent; body lists every threshold's band
• Re-posts only when the forecast goes up — dedup ID WPC-PWPF-Day{n}-{Snow|Ice}-{validDate}-ge{thr}-p{band}; the service checks posted_alerts.txt for any higher-threshold or same-threshold-higher-band ID before returning an alert, so unchanged/downgraded 00Z→12Z cycles are silent and restarts don't re-post
• Severity: snow 1–2" Minor / 4–6" Moderate / 8" Severe / 12"+ Extreme; ice 0.01 / 0.10 / 0.25 / 0.50 likewise
• New IncludePwpf per-platform flag (default true); IsPwpf/PwpfThreshold/PwpfBand on NwsAlert
• Map image: same zoomed Mapbox area map as NWS alerts — the triggering probability ring is unioned, clipped to a padded box around the configured zones, and set as alert.GeometryJson so MapService's existing Mapbox path draws it like a warning polygon. Falls back to WPC's static CONUS gif when Mapbox isn't configured.
• No new NuGet packages

Why PWPF and not the WWD KML/shapefiles

The forecaster-drawn /kml/winwx/ and /shapefiles/ww/ products only exist for ≥4/8/12" — no ≥1" product. The PWPF contour KMZ has 1/2/4/6/8/12/18" plus freezing rain, in plain KML. Details in #33.

Test plan

• dotnet test — 135 pass (27 new: KML parse, valid-window regex, FindBand nesting, file naming, threshold parsing, BuildAlert, severity, overlay clipping, image URL fallback)
• Parser run once against a real downloaded prb_24hsnow_ge01_f024_cntr_latest.kmz (Sept — one 1% ring over the Canadian Rockies) — valid window + polygon parse, point-in-ring correct
• Live: enable Pwpf on the CI/CD instance once there's an actual snow forecast and confirm one post per day/type, the zoomed Mapbox overlay renders, and no re-post on an unchanged cycle

Docs

README feed table + feed-list mentions + Mapbox note, docs/TECHNICAL.md Configuration Reference (Pwpf block, IncludePwpf row) + new "WPC Winter Weather Outlook (PWPF) — How It Works" section, CHANGELOG, CLAUDE.md feed/interface lists.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UokAERat9fVzmcmsfJzsJF