Anchor the gallery's assets to the tree, not to the page directory

The archived gallery resolved ASSETS as BASE + 'assets' — this PAGE's own
directory. That was correct only while the gallery WAS the root page, where the
two coincide. Since it moved to /__gallery it asked for /__gallery/assets/...
Measured on the built tree with the page at /__gallery/:

    /__gallery/assets/vendor/vue/vue.global.prod.js ... 404
    /doxa-maps/assets/vendor/vue/vue.global.prod.js ... 200

Every leaf page already agrees where assets live: a leaf at
/<tree>/<bundle>/<profile>/ bakes assetsBase "../../assets" -> /<tree>/assets.
So anchor to the tree root, which TREE_URL already names.

This degraded QUIETLY rather than blanking, which is why it outlived the move:
loadVue() falls back to the jsDelivr CDN, so the page kept working while
silently losing the portable / offline / no-CDN property the vendored copy
exists to provide. After the fix the built page loads Vue from
/doxa-maps/assets/vendor/vue/ and reports zero 4xx/5xx.

TREE_URL moves above ASSETS because ASSETS now depends on it; both are `var`,
so the old order would have yielded the string "undefined/assets" rather than
an error. Its explanatory comment moves with the declaration.

The dev server passes assetsBaseUrl explicitly, so this is the built-tree half
of the fix its comment already documents for dev.

internal/generate-gallery.mjs

@@ -374,13 +374,6 @@ ${dev ? LADDER_CSS : ''}</style>
     var p = location.pathname || '/';
     return /\.[a-z0-9]+$/i.test(p) ? p.replace(/[^/]*$/, '') : p.replace(/\/*$/, '') + '/';
   })();
-  var ASSETS = ASSETS_BASE || new URL(BASE + 'assets', location.origin).href.replace(/\/+$/, '');
-  var REG = BAKED, active = null, seq = 0, loaded = {}, fetchedToken = '';
-  var narrow = function () { return window.matchMedia && window.matchMedia('(max-width:700px)').matches };
-
-  // Manifest refs are relative to the tree root; this page sits one level above it. Refs are
-  // anchored to BASE (above) so they resolve the same with or without a trailing slash.
-  function rel(u) { return /^([a-z]+:|\/)/i.test(u) ? u : BASE + TREE + '/' + u.replace(/^\.\//, '') }
   // ★ THE TREE ROOT AS ONE ABSOLUTE URL — derive it ONCE, never re-join TREE by hand.
   // TREE is PAGE-RELATIVE and its value differs per surface ('../doxa-maps' in the built tree,
   // '__plugin-gallery' in dev), so a bare TREE + '/x' join is only accidentally right. It was right
@@ -397,6 +390,25 @@ ${dev ? LADDER_CSS : ''}</style>
   // the base URL lets the URL parser apply the right rule per shape: a rooted TREE replaces the
   // path, a '../' TREE climbs out of /__gallery/, and a bare name resolves beside it.
   var TREE_URL = new URL(TREE, new URL(BASE, location.href)).href.replace(/\/+$/, '');
+  // ASSETS live INSIDE the tree (public/<tree>/assets), which is what every leaf page points at:
+  // a leaf at /<tree>/<bundle>/<profile>/ bakes assetsBase "../../assets" -> /<tree>/assets. So
+  // anchor to TREE_URL, not to this PAGE's directory.
+  // This was BASE + 'assets' — correct only while the gallery WAS the root page, where the two
+  // coincide. Since the gallery moved to /__gallery it asked for /__gallery/assets/... MEASURED
+  // on the built tree with the page at /__gallery/:
+  //     /__gallery/assets/vendor/vue/vue.global.prod.js ... 404   (what the page asked for)
+  //     /doxa-maps/assets/vendor/vue/vue.global.prod.js ... 200   (where it lives)
+  // It degraded quietly rather than blanking — loadVue() falls back to the jsDelivr CDN — so the
+  // page still worked while silently losing the portable/offline/no-CDN property the vendored
+  // copy exists to provide. The dev server passes assetsBaseUrl explicitly, so this is the
+  // built-tree half of the same fix its comment already documents.
+  var ASSETS = ASSETS_BASE || TREE_URL + '/assets';
+  var REG = BAKED, active = null, seq = 0, loaded = {}, fetchedToken = '';
+  var narrow = function () { return window.matchMedia && window.matchMedia('(max-width:700px)').matches };
+
+  // Manifest refs are relative to the tree root; this page sits one level above it. Refs are
+  // anchored to BASE (above) so they resolve the same with or without a trailing slash.
+  function rel(u) { return /^([a-z]+:|\/)/i.test(u) ? u : BASE + TREE + '/' + u.replace(/^\.\//, '') }
   function qp(name) { try { return new URLSearchParams(location.search).get(name) || '' } catch (e) { return '' } }
   function hint(msg, err) { var slot = slotEl(); var d = document.createElement('div'); d.className = 'hint' + (err ? ' err' : ''); d.textContent = msg; slot.innerHTML = ''; slot.appendChild(d); return d }
   function assign(t) { for (var i = 1; i < arguments.length; i++) { var s = arguments[i]; for (var k in s) if (Object.prototype.hasOwnProperty.call(s, k)) t[k] = s[k] } return t }