feat(data): make the site correct for 2027+ seasons

[Persblack]

Why

The site assumed a single festival year everywhere. A 2027 dataset would not have failed — it would have quietly lied: the calendar grouped by month index alone, so July 2026 and July 2027 merged into one section labelled July 2026; the analytics histogram summed both years into the same bars; the month slider matched every year with no way to scope one; five copy strings hardcoded 2026. Independently, every festival date went through new Date("2026-07-03") — UTC midnight read back with local getters, which shifts the day for anyone west of Greenwich and mis-buckets month-boundary festivals. tsc --noEmit was already red on three errors, because no script ran it.

Decision (agreed up front): rolling window — the default view is everything from today forward, whatever year it lands in, with year chips to narrow. Adding a season is a file drop; src/data/festivals*.json is globbed, merged and de-duplicated by id, so 2028 needs no code change.

What changed

Data layer (new)

• lib/festival-normalize.ts — pure validator. Drops only rows that cannot be identified or placed in time (missing id/name, unparseable start date, duplicate id); repairs everything else and reports each repair at build time. Fails the build above a 5% drop ratio, so a gutted scrape cannot ship as a stub site.
• lib/festival-data.ts — build-time-only loading seam (never imported from a React island, which would bundle the 1.1 MB dataset). year is derived from start_date, never trusted from the file.
• lib/dates.ts — local-midnight parsing, year-qualified YYYY-MM bucket keys, cross-year ranges (Dec 30, 2027 - Jan 2, 2028), rejects non-dates instead of producing Invalid Date.
• lib/guards.ts — canonical isRecord / hasCoordinates.
• Festival.latitude/longitude/website are nullable, because the data already was (28 rows without coordinates, 47 without a usable URL). Map, Google Maps link, share sheet and recommendations now guard instead of rendering null,null.

Views

• lib/filters.ts — one filter/sort/group implementation replacing four divergent copies of the same predicate chain; prebuilt lowercase search index instead of lowercasing ~8k lineup strings on every keystroke.
• Season chips in FilterBar; calendar sections, jump navigation and planner timeline markers carry the real year; analytics draws one <Area> per season.
• All year-specific copy derives from the data; footer states dataset size, span and freshness.
• Country flags derive from the ISO code (regional indicator symbols) instead of a 9-entry table, so a new country renders without a code change.
• Leaflet popup HTML escapes scraped names/cities.

Gates

• npm run typecheck, npm run test, npm run check; 63 unit tests over dates, filters and the validator. Tests do not read the gitignored dataset.

Dependencies

Dev-only: vitest, @types/node. No env vars, no migrations.

Review checklist

• Rolling-window default is the intended UX (past festivals hidden until "Show past festivals")
• 5% drop ratio is the right build-failure threshold for scraper regressions
• Analytics deliberately covers the whole dataset (past included) while the homepage stats cover only upcoming — the subtitle says "all N festivals"
• Pre-existing any at the Leaflet/recharts boundaries left untouched (out of scope)

Evidence

$ npm run check
tsc --noEmit                       exit 0   (was exit 2 on 3 errors before this branch)
vitest run                         3 files, 63 tests passed
astro build                        7 pages, exit 0
[festival-data] kept 916/916 rows from ../data/festivals.json (2026: 916)
  repaired 47x no usable website URL
  repaired 28x missing or implausible coordinates (excluded from map)

Verified against a synthetic second season (150 shifted rows + 7 deliberate defects) dropped into src/data/festivals_2027.json:

[festival-data] kept 1071/1073 rows from ../data/festivals.json, ../data/festivals_2027.json (2026: 916, 2027: 155)
  dropped 2:
    - defect-no-date-2027: unparseable start_date ("summer 2027")
    - row #1072: missing id
  repaired 155x year field disagreed with start_date (derived from start_date)
  repaired 1x unrecognized genre "Shoegaze" (add it to GENRE_CATEGORY_MAP)
  repaired 1x end_date before start_date (clamped to start)
  repaired 1x unknown size (fell back to medium)

Browser-verified (headless Chromium, 1440x1000) with that two-season dataset:

• Calendar: separate June 2026 … December 2026 / March 2027 … December 2027 sections, jump nav Jun '26 … Dec '27, header badge 2026-2027 Festivals, footer 1071 festivals · 2026-2027 · Data updated 14 Feb 2027
• Season chips All years / 2026 / 2027; clicking 2027 narrows 266 → 155 and every section header is 2027; active chip rgb(59,130,246) vs muted rgb(37,37,37)
• Analytics: 2 <Area> series with a 2026/2027 legend
• Map: 11 festivals without coordinates aren't shown, 236 markers
• Timeline: 570-day span with Jul '26 … Dec '27, Jan '28 markers — no duplicate bare Jan
• Cross-year festival renders Dec 30, 2027 - Jan 2, 2028
• No Invalid Date / NaN on any of the 7 routes

With the synthetic file removed, single-year output is unchanged: no season chips, bare month names in the jump nav, one chart series, 2026 Festivals badge.