Join a people group to its UPG-100 zone, with admin names folded

The UPG-100 lists are defined by admin region and the people-group tables are
keyed by group, so the join runs through the province assignment. The two files
disagree on spelling: the zone table writes `gujarat`, the province table writes
`Gujarāt`, and the same for Karnātaka, Tamil Nādu, Uttarākhand, Jammu and
Kashmīr and Haryāna.

Matching on toLowerCase alone joins 514 of 1009 rows across 9 zones. Folding the
combining marks joins 1009 of 1009 across 12 zones.

The 50.9% is the number worth naming: a legend built on it would have shown half
the groups and three fewer zones, and nothing on screen would have said so —
every zone still looks populated, only smaller. The fold lives at the lookup
rather than as extra spellings in the zone table, so a future source spelling
cannot reopen it.

plugin-designs/gospel-internet/src/upgZones.js

@@ -0,0 +1,57 @@
+/**
+ * upgZones.js — join a people group to its UPG-100 zone.
+ *
+ * THE CHAIN, and why it needs three files. UPG-100 lists are defined by ADMIN REGION
+ * ("Tibet Autonomous Region", "Gujarat"); the people-group tables are keyed by group.
+ * They share no id, so the bridge is the province assignment:
+ *
+ *   admin-to-list.json   admin name        -> zone id
+ *   gospel-by-province-groups.json  group  -> province name
+ *
+ * ★ ADMIN NAMES ARE MATCHED WITH DIACRITICS FOLDED, and that is load-bearing rather than
+ * defensive. The two files disagree on spelling: the zone table writes `gujarat`, the
+ * province table writes `Gujarāt`; likewise Karnātaka, Tamil Nādu, Uttarākhand, Jammu and
+ * Kashmīr, Haryāna. Matching on `.toLowerCase()` alone resolves 514 of 1009 rows across 9
+ * zones. Folding the combining marks resolves 1009 of 1009 across 12 zones.
+ *
+ * That 50.9% is the dangerous number, not the 100%: a legend built on it would have shown
+ * half the groups and three fewer zones with NOTHING ON SCREEN SAYING SO — every zone would
+ * still have looked populated, just smaller. Fold here rather than adding spellings to the
+ * zone table, so a future source spelling cannot reopen it.
+ */
+
+/** Lowercase and strip combining marks, so `Gujarāt` and `gujarat` are the same key. */
+export function foldAdmin(name) {
+  return String(name ?? '')
+    .normalize('NFD')
+    .replace(/\p{Mn}/gu, '')
+    .trim()
+    .toLowerCase()
+}
+
+/** Build a folded lookup from the zone table's `admin name -> zone id` map. */
+export function zoneIndex(adminToList) {
+  const ix = new Map()
+  for (const [admin, zone] of Object.entries(adminToList || {})) ix.set(foldAdmin(admin), zone)
+  return ix
+}
+
+/** The zone id for a province name, or null when the province is in no UPG-100 list. */
+export function zoneFor(ix, province) {
+  return ix.get(foldAdmin(province)) ?? null
+}
+
+/**
+ * group key -> zone id, from the province bridge rows.
+ * `key` decides the join key into whatever table the caller holds; the default
+ * `(name, iso3)` is the pair that matches near-neighbors at 994/1009.
+ */
+export function zonesByGroup(adminToList, provinceRows, key = (r) => `${r.people_group}|${r.iso3}`) {
+  const ix = zoneIndex(adminToList)
+  const out = new Map()
+  for (const r of provinceRows || []) {
+    const z = zoneFor(ix, r.province)
+    if (z) out.set(key(r), z)
+  }
+  return out
+}