Heat-grid anomaly toggle: "vs 1991–2020 normal"

[Ashwask]

What

Adds an Absolute °C / vs normal toggle to the 0.25° heat grid. In vs normal mode each cell is colored by its anomaly — today's temperature minus the cell's 1991–2020 monthly normal (WMO-standard 30-year baseline) — on a diverging blue→red scale (blue = cooler than normal, red = hotter).

Why

We show absolute temperature everywhere, so a user can't tell whether 38 °C in Delhi in June is normal or alarming. The comparison site climate.thisindianlife.today gets its whole value from anomaly-vs-baseline framing ("the wobble is weather, the drift is climate"). This brings that "is this unusual?" lens to our live spatial map while keeping the real-time grid as our differentiator.

Changes

Frontend — index.html

• Sidebar segmented toggle (Absolute °C / vs normal); hidden gracefully if the baseline file is absent.
• Diverging anomaly color scale (ANOM_BANDS / colorForAnomaly, ColorBrewer RdBu) + a swappable legend.
• renderGridLayer(mode) recolors the grid in place; popups now show today's temp, the cell's monthly normal, and the signed anomaly (e.g. +2.3 °C vs Jun normal).

Data builder — build_grid_normals.py (new, static / one-time)

• Builds a 1991–2020 monthly climatology per grid cell from Open-Meteo's archive (ERA5 temperature_2m_mean).
• Archive calls are weighted ~50 units/point, so the free tier can't fetch all 4,647 cells directly — it fetches a coarse 1.0° lattice and IDW-interpolates to the 0.25° grid (monthly normals are spatially smooth).
• Waits out hourly and daily rate limits; resumable via a JSONL partial. Not added to the 3-hourly refresh cron.

Docs — README (feature, builder, static data file, local run step) + NOTICE (ERA5/Copernicus attribution).

Follow-up in this PR

india_grid_normals.json (the generated baseline) is not in this commit yet — the Open-Meteo daily quota is exhausted, so the rate-limited fetch resumes after the daily reset and the data file will be pushed to this branch when complete. Until then the toggle is gracefully hidden, and the absolute map is unchanged. Do not merge until the data file lands.

Verification done

• JS syntax-checked (node --check); builder parses clean.
• Anomaly decode/lookup/color logic unit-verified: +5 °C→orange, +0.5→neutral, −4→blue, missing cell→grey N/A.
• Builder smoke-tested against the archive API: Delhi June normal ≈ 33 °C, Ladakh deep-negative in winter — correct.
• Pending once data lands: in-browser toggle + legend swap + popup check.

🤖 Generated with Claude Code

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