iter 22b.1.2: workspace registry skeleton + coherence checks

Adds the workspace-global typeclass instance registry per Decision 11
"Resolution and monomorphisation". Built at the end of load_workspace
after the DFS over imports completes; keyed by (class-name, type-hash)
where type-hash uses the new canonical::type_hash (16-hex prefix,
parallel to def_hash and module_hash).

Three coherence checks fire from build_registry, each surfacing as a
distinct WorkspaceLoadError variant:

- OrphanInstance: `instance C T` not in C's or T's defining module.
- DuplicateInstance: two entries share the same (class, type-hash) key.
- MissingMethod: instance omits a required (non-default) method.

The CLI's workspace_error_to_diagnostic is extended with the three new
codes (orphan-instance / duplicate-instance / missing-method).

All existing Workspace { ... } construction sites get the new
`registry` field, defaulting to Registry::default() at synthetic /
test-only paths and threading through ws.registry.clone() at codepath
sites that already hold a real workspace.

Includes hash-stability regression tests (iter22b1_schema_extension_*
and iter22b1_classdef_empty_optionals_hash_stable) and the empty-
registry positive test against examples/sum.ail.json. Test count:
288 → 291 (3 new tests, all pass).

crates/ailang-codegen/src/lib.rs

@@ -195,6 +195,12 @@ pub fn emit_ir(m: &Module) -> Result<String> {
         entry: m.name.clone(),
         modules,
         root_dir: std::path::PathBuf::from("."),
+        // Iter 22b.1: emit_ir is the single-module shortcut. The
+        // empty registry is fine for 22b.1 because codegen does not
+        // yet read it; once 22b.3 monomorphisation runs, the queue
+        // is built from class-method-call sites in already-typechecked
+        // module bodies, not from the registry.
+        registry: ailang_core::workspace::Registry::default(),
     };
     lower_workspace(&ws)
 }
@@ -251,6 +257,11 @@ fn lower_workspace_inner(ws: &Workspace, alloc: AllocStrategy) -> Result<String>
             .map(|(k, m)| (k.clone(), ailang_core::desugar::desugar_module(m)))
             .collect(),
         root_dir: ws.root_dir.clone(),
+        // Iter 22b.1: pass the registry through unchanged. The desugar
+        // pass does not touch class/instance defs (see desugar.rs:
+        // 22b.1 passthrough), so the registry built at load time
+        // remains valid against the desugared modules.
+        registry: ws.registry.clone(),
     };
     let ws = &ws_owned;
     let mut header = String::new();
@@ -2837,6 +2848,7 @@ mod tests {
                 x
             },
             root_dir: std::path::PathBuf::from("."),
+            registry: ailang_core::workspace::Registry::default(),
         };
         let ir_rc = lower_workspace_with_alloc(&ws, AllocStrategy::Rc).unwrap();
         assert!(