feat(weather): weather impact summaries (outdoor/driving/allergy)

[Orinks]

Summary

Closes #324

Adds rule-based weather impact summaries derived from existing data — no new API dependencies required.

• Outdoor: comfort bands from feels-like (or actual) temperature, UV index protection note, active-precipitation warning
• Driving: visibility thresholds (near-zero / very low / reduced), precipitation-type detection (ice > snow > thunderstorm > rain), near-freezing black-ice warning, wind severity bands
• Allergy: pollen category/index bands with primary allergen name, wind dispersion note when wind ≥ 15 mph with moderate+ pollen, air quality modifier for Unhealthy and worse AQI

Impact summaries appear as labelled metrics in current conditions and forecast sections. Screen readers get concise, consistently-structured lines like Impact: Outdoor: Comfortable - good conditions for outdoor activities.

What changed

File	Change
src/accessiweather/impact_summary.py	New module — pure rule-based logic, fully documented
src/accessiweather/display/weather_presenter.py	Added impact_summary field to CurrentConditionsPresentation and ForecastPresentation
src/accessiweather/display/presentation/current_conditions.py	Calls build_impact_summary, appends three Metric rows, attaches ImpactSummary to presentation
src/accessiweather/display/presentation/forecast.py	Calls build_forecast_impact_summary from first daily period, attaches to ForecastPresentation
tests/test_impact_summary.py	76 unit tests covering all rule branches and integration with presentation builders
CHANGELOG.md	User-facing entry under Unreleased > Added

Test plan

• pytest tests/test_impact_summary.py — 76 tests, all passing
• Full suite pytest tests/ — 3110 passed, 4 skipped, 0 failures
• Pre-commit hooks (ruff lint + format) pass

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