Revert #6: keep global hierarchy module-local

Hoisting _globalHierarchy onto globalThis[Symbol.for(…)] was
over-defensive. The playground bug we used as precedent was fixed by
unifying URLs, not by papering over duplicate loads with shared
state. Normal ESM semantics — one module instance per realm per URL —
is the correct behavior, and every other module-level state in squint
(including core.js's _metaSym) lives that way. cljs.core's
*global-hierarchy* is module-scoped for the same reason.

A user who ends up with two instances of multi.js under different
URLs almost certainly has two versions of squint in their graph;
sharing state across versions via globalThis would hide real
version-mismatch bugs instead of surfacing them.

Move #6 from Done to Wontfix in todo.md with the reasoning.

Author: borkdude

src/squint/multi.js

@@ -19,16 +19,10 @@ export function make_hierarchy() {
   };
 }
 
-// Store the global hierarchy on globalThis under a Symbol.for key so all
-// instances of this module (e.g. if the bundle ends up loaded twice
-// under different URLs — see the playground dual-module episode) share
-// one hierarchy. Without this, a `derive` through one instance would be
-// invisible to `isa?` queries issued through the other.
-const GH_KEY = Symbol.for('squint.multi.hierarchy');
+let _globalHierarchy = null;
 function gh() {
-  return globalThis[GH_KEY] ?? (globalThis[GH_KEY] = make_hierarchy());
+  return _globalHierarchy ?? (_globalHierarchy = make_hierarchy());
 }
-function setGlobalHierarchy(h) { globalThis[GH_KEY] = h; }
 
 function _isa(h, child, parent) {
   if (_EQ_(child, parent)) return true;
@@ -89,9 +83,8 @@ export function derive(a, b, c) {
     // invalidate, so in-place mutation would leave stale cached
     // resolutions in place (e.g. a subsequent derive can introduce
     // ambiguity that the cache would otherwise hide).
-    const next = cloneHierarchy(gh());
-    _deriveInto(next, a, b);
-    setGlobalHierarchy(next);
+    _globalHierarchy = cloneHierarchy(gh());
+    _deriveInto(_globalHierarchy, a, b);
     return null;
   }
   const next = cloneHierarchy(a);
@@ -122,7 +115,7 @@ export function underive(a, b, c) {
     }
   }
   if (c === undefined) {
-    setGlobalHierarchy(rebuildFromPairs(pairs));
+    _globalHierarchy = rebuildFromPairs(pairs);
     return null;
   }
   return rebuildFromPairs(pairs);

todo.md

@@ -14,10 +14,17 @@ Captured from PR feedback on the `multimethods` branch.
   change as potentially breaking and explains the migration.
 - **#5** `defmulti` now auto-wraps a plain `(make-hierarchy)` passed as
   `:hierarchy` so `.deref()` dispatch doesn't crash. Regression test.
-- **#6** `_globalHierarchy` moved onto
-  `globalThis[Symbol.for('squint.multi.hierarchy')]` so dual module
-  loads (e.g. npm + CDN, symlink quirks) share one hierarchy. No
-  user-facing test — observable only by reaching into internals.
+## Wontfix
+
+- **#6** `_globalHierarchy` dual-module trap. The playground fix for
+  the `_metaSym` analogue unified URLs rather than hoisting state, and
+  normal ESM semantics (one instance per realm per URL) is the
+  correct behavior — `cljs.core`'s `*global-hierarchy*` is
+  module-scoped for the same reason. A user who ends up with two
+  instances of `multi.js` under different URLs has a bundler/config
+  problem (likely two versions of squint in the graph); sharing state
+  across versions via `globalThis` would hide real version-mismatch
+  bugs instead of surfacing them. Module-local state stays.
 
 ## Defer (file as follow-up issue)