A shared Go client for the api.forecast.solar PV-production forecast API. It implements the API's operational requirements once so the services that consume forecast.solar don't each re-implement (and mis-implement) them:

• 429 retry-at self-pause (typed *RateLimited) with a minimum backoff floor — no "infinite 429 loop".
• Accept / User-Agent headers, ?time=seconds epoch timestamps.
• Endpoints: Estimate (live weather), ClearSky (no-cloud ceiling), Historic (/history — historic-average "typical" production, paid tiers only), and KeyInfo (/info).
• Multi-plane batching into ⌈planes / planes-per-call⌉ requests, summed per timestamp.
• Planes-per-call detection: a keyed client lazily reads /{key}/info (result.strings) on its first fetch — authoritative, incl. exact plan name (account, so Enterprise is identifiable) and subscription expiry (until). Falls back to inferring from the rate-limit limit when /info is unavailable. Keyless (free public tier) uses the limit inference.
• Optional per-call modifiers (functional options, all default off): WithInverterKW, WithActualW (both cap/adjust the summed output per request → guarded against multi-plane batch splits), WithResolution, WithDamping, WithHorizon, and a WithParam escape hatch.
• Optional per-client MaxCallsPerHour limiter, deterministic Jitter, and a NOAA SunUp helper.
• Secret hygiene: the API key lives in the URL path, so the client redacts it from returned errors and the retained /info body.

The Client is safe for concurrent use and is designed to be shared across a fleet behind one key (its 429 pause / plan / rate-limit state is per-client, which matches how forecast.solar rate-limits). Scheduling, caching, and any fleet-wide (store-persisted) call budget stay with the caller.

c := &forecastsolar.Client{Key: os.Getenv("FSOLAR_KEY")} // Key=="" → keyless free tier
pts, meta, err := c.Estimate(ctx, lat, lon, []forecastsolar.Plane{{Dec: 30, Az: -15, Kwp: 7.5}})
// pts: []Point{Ts, Watts, WhPeriod}; meta.Calls: API calls made (for budget bookkeeping)

Consumed by ems-datastore (paid key, fleet dispatch) and blackstarems (keyless fallback).