generics: more inference coverage, native TypeError, class-method must-specify, deep unbound recursion

Tests (bex + python):
- heterogeneous array element unification (elem_type)
- unbound-instance forcing-formal recovery (ContainerShapes, GenericRecursive,
  nested GenericPair) — now incl. FULLY-unbound deep nesting
- concrete baml_sdk types in covariant joins (enum + class)
- partial _types= noted as unusual; prefer inference / subscript

Inference errors -> native TypeError:
- route EngineError::TypeMismatch to a structured baml.errors.TypeMismatch
  (bridge_cffi) instead of an opaque SdkPanic; map to Python TypeError in
  proto.py; assert type + message on every must-specify / conflict case
- add baml.errors.TypeMismatch stdlib class; refresh builtin/HIR/TIR/bytecode
  snapshots

Class-method must-specify (rule 3):
- carry generic_param_count on the runtime Class; Gate A demands a method's OWN
  generic params (suffix after the class prefix), so a body-only own var
  (reflect_t<T>()) is rejected, matching the free-function analogue

Deep unbound recursion:
- reconstruct_unbound_instance_args is formal-aware, recursing through nested
  unbound instances so all vars are recovered from leaf field values

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: sxlijin

baml_language/crates/baml_builtins2/baml_std/baml/ns_errors/errors.baml

@@ -80,3 +80,13 @@ class GenericSdkError {
 class CompilationError {
   message string
 }
+
+/// A value/type mismatch at the call boundary — a caller passed an argument
+/// that doesn't fit the callee's (possibly inferred) type, a generic `TypeVar`
+/// could not be inferred and must be specified, or repeat occurrences of a
+/// `TypeVar` have no consistent binding. Synthesized host-side from
+/// `EngineError::TypeMismatch`; each host SDK surfaces it as its native
+/// type-error (Python `TypeError`).
+class TypeMismatch {
+  message string
+}

baml_language/crates/baml_cli/src/snapshots/baml_cli__describe_command_tests__render_builtin_package_listing.snap

@@ -86,6 +86,7 @@ class            baml.errors.DevOther             <builtin>/baml/ns_errors/error
 class            baml.errors.HostCallable         <builtin>/baml/ns_errors/errors.baml:53
 class            baml.errors.GenericSdkError      <builtin>/baml/ns_errors/errors.baml:74
 class            baml.errors.CompilationError     <builtin>/baml/ns_errors/errors.baml:80
+class            baml.errors.TypeMismatch         <builtin>/baml/ns_errors/errors.baml:90
 class            baml.errors.StackFrame           <builtin>/baml/ns_errors/stack_trace.baml:2
 class            baml.errors.StackTrace           <builtin>/baml/ns_errors/stack_trace.baml:9
 class            baml.fs.File                     <builtin>/baml/ns_fs/fs.baml:6

baml_language/crates/baml_compiler2_emit/src/lib.rs

@@ -869,6 +869,7 @@ pub fn generate_project_bytecode_with_opt(
                 type_tag,
                 ty_attr: TyAttr::default(),
                 has_cleanup,
+                generic_param_count: class_data.generic_params.len(),
             })));
             // Register with fully-qualified name for inter-package lookups.
             class_object_indices.insert(fq_name.clone(), class_obj_idx);