iter 22b.2.10: fix — positive green-path test, drop dead let _ = ws

Adds the missing positive counterpart to the no-instance test:
`show 42` with `instance Show Int` PRESENT must typecheck green.
This guards against registry-keying or threading bugs that would
otherwise sneak past the negative-only test (an empty registry at
typecheck time, or a key shape mismatch, would silently fire
no-instance even when the instance exists).

Also drops the dead `let _ = ws;` and its stale comment in
`check_in_workspace` — the line above already consumes
`ws.registry`, so the explicit drop is redundant. The remaining
comment is folded into the registry-threading comment so the
"registry is the only thing threaded" rationale is preserved.

examples/test_22b2_instance_present.ail.json

@@ -0,0 +1,40 @@
+{
+  "schema": "ailang/v0",
+  "name": "test_22b2_instance_present",
+  "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" } }
+      }]
+    },
+    {
+      "kind": "fn",
+      "name": "main",
+      "type": {
+        "k": "fn", "params": [], "ret": { "k": "con", "name": "Str" },
+        "effects": []
+      },
+      "params": [],
+      "body": {
+        "t": "app",
+        "fn": { "t": "var", "name": "show" },
+        "args": [{ "t": "lit", "lit": { "kind": "int", "value": 42 } }]
+      }
+    }
+  ]
+}