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.

crates/ail/tests/typeclass_22b2.rs

@@ -235,3 +235,26 @@ fn fn_calling_class_method_at_type_without_instance_fires_no_instance() {
         diagnostics.iter().map(|d| &d.code).collect::<Vec<_>>()
     );
 }
+
+/// Property protected: the positive counterpart to `no-instance` — when a
+/// monomorphic fn calls a class method at a fully-concrete type AND a
+/// matching `instance C T` exists in the workspace, the call typechecks
+/// green. The concrete-residual path must look up `(class, type_hash(T))`
+/// in `env.workspace_registry` and find a hit, so no diagnostic fires.
+/// This guards against registry-keying or threading bugs that would
+/// otherwise sneak past the negative-only test: a registry that is empty
+/// at typecheck time, or keyed under the wrong shape, would silently fire
+/// `no-instance` even when the instance is present. Fixture: `main` calls
+/// `show 42` with `instance Show Int` declared in the same workspace.
+#[test]
+fn fn_calling_class_method_at_type_with_instance_typechecks_green() {
+    let entry = examples_dir().join("test_22b2_instance_present.ail.json");
+    let ws = ailang_core::workspace::load_workspace(&entry)
+        .expect("workspace loads");
+    let diagnostics = ailang_check::check_workspace(&ws);
+    assert!(
+        diagnostics.is_empty(),
+        "expected no diagnostics — instance Show Int is present, registry lookup must hit; got: {:?}",
+        diagnostics
+    );
+}