feat(rain): nowcast enhancements + tests (amounts, capitals, snow/ice, !snow, probability, changeover)

[rlwilliamson-dev]

Summary

Builds on the merged minute-level rain/snow nowcast (#193) with ten enhancements plus end-to-end, proactive, and live-smoke test coverage. Everything new is config-gated or additive — existing deployments are unchanged by default.

Enhancements

• Amount estimate — (est 0.2 in) on !rain/!nowcast and the proactive push; sums Open-Meteo per-bucket precip over the episode.
• Real snow depth — snow shows accumulation via Open-Meteo snowfall (cm), not liquid-equivalent; freezing rain tagged in ice.
• Region defaulting — a bare country / US state resolves to its capital with a heads-up (!rain france → Paris, France), via a self-contained modules/region_capitals.py (no new dependency).
• Location dedupe — join_location() collapses "Spain, Spain" and city-states.
• !snow alias + neutral !nowcast — each looks for its own precip family across the window, otherwise falls back with a cross-type line ("No snow, but rain …").
• Keyword-aware help — help rain / help snow describe the right mode.
• Probability — fetches 15-min precipitation_probability, shows (…, 70%), and gates the proactive incoming alert at >= rain_nowcast_min_probability (default 50) to cut false alarms.
• Rain ↔ snow changeover — a single line when the window holds both families.
• Borderline temperature tag — shown only in the 30–38 °F band, where precip type is in doubt.
• Short-lived series cache — shared by the command and the proactive poll to avoid duplicate Open-Meteo fetches.

Tests

• test_rain_command_e2e.py — drives RainCommand.execute() end to end and asserts the exact rendered reply across dry / incoming / raining, snow depth, cross-type mismatch, changeover, ice, temp tag, region capitals, config toggles, DM budget, and keyword-aware help.
• test_rain_proactive_e2e.py — drives Weather_Service._check_rain_nowcast() for the probability gate, snow depth, rain-ending notice, and once-per-episode dedup.
• test_rain_live_smoke.py — opt-in (RAIN_LIVE_SMOKE=1) live Open-Meteo check that the API still serves the fields we depend on; skipped in CI.
• Shared scaffolding in tests/unit/_rain_harness.py. Rain suite: 93 passing, +2 opt-in live-smoke skipped.

Config (all optional; defaults preserve current behavior)

[Rain_Command]: show_amount, amount_unit, show_probability, show_temp, cache_seconds, zip_city_lookup
[Weather_Service]: rain_nowcast_min_probability, rain_nowcast_cache_seconds, rain_nowcast_show_amount, rain_nowcast_amount_unit

Notes

ruff + mypy clean (against the pinned ruff 0.15.15). Deployed and running in production for several days.

[rlwilliamson-dev]

Note: #196 (NWS gridpoint precip source) is now stacked on top of this PR. Merging this one first lets #196's diff collapse to just its own change, conflict-free — no rebase needed on either side.