Pin the three layers that make Show on map work on the first click

The composite's first "Show on map" did nothing and only a second click
selected the zone: the list pane dispatched an event at a map pane whose
listener attaches in onMounted, and the pane mounts lazily. That is
fixed on main already -- this is the regression gate it never had.

The fix is three mechanisms, so the gate asserts them separately rather
than asking one "does it work" question that would go green on two of
three. Delivery: the request is written as an attribute before the event
is dispatched, because an element dispatch has no replay and an
attribute is state that survives element upgrade. Arrival: the pane
reads that attribute on mount and observes it afterwards, covering
requests that predate the pane and requests that follow it. Effect: an
arrived request that cannot apply -- because the legend node is not
there yet, or because onLegendSelect refused while the tapestry layer
was down -- is retained and drained at the readiness signal, with the
drain re-checking which request it is draining so a newer selection
cannot be overwritten by an older one.

Mutation-tested per layer, and one mutant escaped first time: there are
TWO retain sites, node-not-found and applied-but-no-selection, and a
single pattern match stayed green while one of them was deleted. They
fail at different layers -- arrival before data versus arrival before
readiness -- so the gate now counts them.

Source-level by design: the race is a lifecycle ordering between two
custom elements, which a DOM-less runner cannot stage. The rendered
check is the browser seat's; what this pins is that none of the three
mechanisms quietly disappears.

internal/check-select-zone-race.mjs

@@ -0,0 +1,92 @@
+#!/usr/bin/env node
+/**
+ * check-select-zone-race — "Show on map" works on the FIRST click, not the second.
+ *
+ * THE DEFECT (PRODUCTION-MATRIX:422, measured by rega): the composite's list pane dispatches
+ * 'doxa:select-zone' at the map pane, but the map pane attaches its listener in onMounted and the
+ * pane mounts lazily. So the first click landed before any listener existed, did nothing, and only
+ * a SECOND click selected the zone. A control you must press twice reads as broken.
+ *
+ * ★ IT IS A THREE-LAYER DEFECT AND THE FIX HAS THREE PARTS, so this gate asserts them SEPARATELY —
+ * removing any one of them alone must fail by name. A single "does it work" assertion would go
+ * green on two of three and let the third rot back in:
+ *   1 DELIVERY    an element dispatch has no replay, so the request is written as an ATTRIBUTE
+ *                 first (state survives custom-element upgrade); the event stays as the live path.
+ *   2 ARRIVAL     the pane reads that attribute on mount AND observes it afterwards, so a request
+ *                 that predates the pane and one that follows it both arrive.
+ *   3 EFFECT      onLegendSelect bails while the map's tapestry layer is down, so an arrived
+ *                 request is RETAINED (_pendingZoneId) and drained at the readiness signal.
+ *                 Delivery and arrival both succeeding still lost the selection without this.
+ *
+ * Source-level and headless: the race is a lifecycle ordering between two custom elements, which a
+ * DOM-less runner cannot stage and which the browser seat measures end to end (rega owns that).
+ * What this pins is that none of the three mechanisms quietly disappears.
+ *
+ * Exit 0 pass · 1 fail.
+ */
+import { resolve, dirname, join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { readFileSync } from 'node:fs'
+
+const HERE = dirname(fileURLToPath(import.meta.url))
+const P = (f) => join(resolve(HERE, '..'), 'plugin-designs', 'my-upg-100-list', f)
+const MFE = readFileSync(P('upg-100-mfe.vue'), 'utf8')
+const LIST = readFileSync(P('upg-100-list.vue'), 'utf8')
+
+const fails = []
+const ok = (c, m) => { if (!c) fails.push(m) }
+
+// ── 1. DELIVERY: the attribute is written, and written BEFORE the event ────────────────────────
+ok(/setAttribute\('select-zone'/.test(MFE),
+  "the composite no longer writes the select-zone ATTRIBUTE — an element dispatch has no replay, so a request that predates the map pane is lost and the first click does nothing")
+ok(/removeAttribute\('select-zone'/.test(MFE),
+  'clearing a selection no longer removes the attribute — a stale attribute would re-apply an old zone when the pane mounts')
+{
+  const iAttr = MFE.indexOf("setAttribute('select-zone'")
+  const iEvt = MFE.indexOf("doxa:select-zone', { detail")
+  ok(iAttr > 0 && iEvt > 0 && iAttr < iEvt,
+    'the event is dispatched BEFORE the attribute is written — record the request, then announce it, or a listener that runs synchronously reads the old value')
+}
+
+// ── 2. ARRIVAL: read on mount, and observed after ──────────────────────────────────────────────
+ok(/getAttribute\('select-zone'\)/.test(LIST),
+  'the map pane never READS select-zone — the composite would be writing state nobody consumes, which looks identical to a fix')
+ok(/attributeFilter:\s*\['select-zone'\]/.test(LIST),
+  'the select-zone MutationObserver is gone — a request made AFTER the pane mounted would never arrive')
+ok(/addEventListener\('doxa:select-zone'/.test(LIST),
+  'the live event path is gone — a map that is already up would wait for a mutation that never comes')
+
+// ── 3. EFFECT: retained while unready, drained at the readiness signal ─────────────────────────
+// ⚠ THE SUBTLEST LAYER. Delivery and arrival can both succeed and the selection still not hold,
+// because onLegendSelect returns early while the tapestry layer is down. rega read that state
+// twice, 13 ms apart, with the correct id.
+// ⚠ TWO DISTINCT RETAIN SITES, and a single /_pendingZoneId = zoneId/ match passes while ONE of
+// them is deleted — measured: removing the not-found retain left the gate green on the other.
+// They are different failures: the legend has no such node YET (arrival before data), versus the
+// node resolved and onLegendSelect refused because the tapestry layer was down (arrival before
+// readiness). Count them.
+{
+  const retains = (LIST.match(/_pendingZoneId\s*=\s*zoneId/g) || []).length
+  ok(retains >= 2,
+    `only ${retains} retain site(s) for an un-applied request; expected both — the node-not-found case AND the applied-but-no-selection case, which fail at different layers and are each discarded silently without their own retain`)
+}
+ok(/if \(_pendingZoneId\)/.test(LIST) && /applySelectZone\(id\)/.test(LIST),
+  'the retained request is never DRAINED at the readiness signal — it would be remembered forever and never applied')
+// The drain must re-check identity: a second request between retain and drain must win.
+ok(/if \(_pendingZoneId === id\)/.test(LIST),
+  'the drain does not re-check which request it is draining — a newer selection made while the map was loading would be overwritten by the older one')
+
+// ── 4. ONE APPLY PATH, TWO TRIGGERS ────────────────────────────────────────────────────────────
+// Both the attribute and the event must end in the same function, or the two paths drift into two
+// different meanings of "select a zone" — the class this repo keeps finding as two copies of a fact.
+{
+  const applies = (LIST.match(/applySelectZone\(/g) || []).length
+  ok(applies >= 3, `applySelectZone is called ${applies} time(s); expected the attribute path, the event path and the drain to share it`)
+}
+
+if (fails.length) {
+  console.error('SELECT-ZONE-RACE: FAIL')
+  for (const f of fails) console.error(`  ✗ ${f}`)
+  process.exit(1)
+}
+console.log('SELECT-ZONE-RACE: PASS — the request is recorded as state before it is announced, read on mount and observed after, retained while the map is unready and drained (identity-checked) when it is ready, with one apply path behind both triggers.')