iter 22b.2.e2e: cross-module class resolution + multi-fn aggregation

Four new E2E tests in typeclass_22b2.rs covering invariants the
per-task fixtures (all single-module, single-fn) did not exercise:

- cross_module_class_method_resolves_and_fires_no_instance:
  module B's call to `show` (declared in module A) reaches the
  no-instance arm, proving Env.class_methods is merged workspace-
  wide and not restricted to the local module.
- cross_module_polymorphic_fn_without_constraint_fires_missing_constraint:
  same merge guarantees the missing-constraint arm sees the residual
  produced by a call to A's class method from inside B.
- cross_module_instance_satisfies_concrete_call: the workspace
  registry built at load time over all modules must populate keys
  visible from B; a concrete call resolves clean cross-module.
- two_fns_each_missing_constraint_produce_two_diagnostics: pins
  check_in_workspace's per-def aggregation — two ill-formed fns
  produce two diagnostics (one per fn), not one.

Six new fixtures (xmod_* trio + two_fns_*).

examples/test_22b2_xmod_classmod.ail.json

@@ -0,0 +1,34 @@
+{
+  "schema": "ailang/v0",
+  "name": "test_22b2_xmod_classmod",
+  "imports": [],
+  "defs": [
+    {
+      "kind": "class",
+      "name": "Show",
+      "param": "a",
+      "methods": [
+        {
+          "name": "show",
+          "type": {
+            "k": "fn",
+            "params": [{ "k": "var", "name": "a" }],
+            "ret": { "k": "con", "name": "Str" },
+            "effects": []
+          }
+        }
+      ]
+    },
+    {
+      "kind": "instance",
+      "class": "Show",
+      "type": { "k": "con", "name": "Int" },
+      "methods": [
+        {
+          "name": "show",
+          "body": { "t": "lit", "lit": { "kind": "str", "value": "n" } }
+        }
+      ]
+    }
+  ]
+}