Effect PM2.5 field service: cached OpenAQ viewport sensors plus point kernel sample

[Jesssullivan]

Linear: https://linear.app/tinyland/issue/TIN-1726/effect-pm25-field-service-cached-openaq-viewport-sensors-plus-point

Problem

PM2.5 viewport loading, cancellation, station mapping, kernel estimation, and transmission AOD handoff are currently spread across page-local Svelte state and helper functions. That makes lifecycle behavior harder to test and makes provenance harder to reuse in popovers and the transmission sheet.

Scope

• Add a higher-level Effect service around OpenAQ viewport fetches and PM2.5 point sampling.
• Return a canonical field-sample object containing estimate, confidence, contributing stations, effective station count, nearest station, numeric/null counts, bandwidth/radius, computedAt, and source.
• Own abort/generation behavior and normalized viewport cache keys inside the service layer.
• Replace page-local station mapping/sample assembly with the service output.

Acceptance

• A map tap produces one typed PM2.5 field sample consumed by PointReadout and TransmissionSheet.
• Abort/generation behavior remains race-safe on rapid pan/tap/layer toggles.
• Existing null-exclusion and sparse-confidence behavior from pm25-diffusion remains intact.
• Unit tests cover cancellation, cache-key normalization, sparse/no-reading outputs, and happy-path sample creation.

Related: #275, #271, #253.