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-check/tests/workspace.rs

@@ -112,6 +112,7 @@ fn invalid_def_name_with_dot_is_reported() {
         entry: m.name.clone(),
         modules,
         root_dir: std::path::PathBuf::from("."),
+        registry: ailang_core::workspace::Registry::default(),
     };
     let diags = check_workspace(&ws);
     assert_eq!(diags.len(), 1, "got: {:?}", diags);
@@ -184,6 +185,7 @@ fn body_errors_accumulate_across_defs() {
         entry: m.name.clone(),
         modules,
         root_dir: std::path::PathBuf::from("."),
+        registry: ailang_core::workspace::Registry::default(),
     };
     let diags = check_workspace(&ws);
 
@@ -288,6 +290,7 @@ fn use_after_consume_on_own_param_is_reported() {
         entry: m.name.clone(),
         modules,
         root_dir: std::path::PathBuf::from("."),
+        registry: ailang_core::workspace::Registry::default(),
     };
     let diags = check_workspace(&ws);
 
@@ -389,6 +392,7 @@ fn consume_while_borrowed_in_sibling_arg_is_reported() {
         entry: m.name.clone(),
         modules,
         root_dir: std::path::PathBuf::from("."),
+        registry: ailang_core::workspace::Registry::default(),
     };
     let diags = check_workspace(&ws);
 
@@ -527,6 +531,7 @@ fn reuse_as_happy_path_in_map_inc_is_linearity_clean() {
         entry: m.name.clone(),
         modules,
         root_dir: std::path::PathBuf::from("."),
+        registry: ailang_core::workspace::Registry::default(),
     };
     let diags = check_workspace(&ws);
     assert!(
@@ -638,6 +643,7 @@ fn reuse_as_shape_mismatch_is_reported_on_cons_to_nil() {
         entry: m.name.clone(),
         modules,
         root_dir: std::path::PathBuf::from("."),
+        registry: ailang_core::workspace::Registry::default(),
     };
     let diags = check_workspace(&ws);
     let shape_diags: Vec<&ailang_check::Diagnostic> = diags