Validate BAST documents against the meta-schema at compile time

Resolves review #003 finding M3.

- Add bast_meta::validate_bast_doc and call it from AlkTypeEngine::compile
  before parsing. Malformed annotations (unknown endian/encoding strings,
  non-integer align/maxLength, missing required properties) now surface as
  AlkTypeError::Schema instead of being silently tolerated by the parser.
- Align the meta-schema TypeRef with the parser: allow inline struct/union/
  enum as TypeRefs (the parser and builder already accepted them; the
  meta-schema and spec did not). Update bast-format.md TypeRef table and
  the BastType doc comment to the seven-form vocabulary.

Verification: cargo test --release (410 pass), cargo clippy --all-targets
-- -D warnings (clean), cargo doc --no-deps (clean), cargo build --target
wasm32-unknown-unknown --release (clean).

src/engine.rs

@@ -126,6 +126,7 @@ impl AlkTypeEngine {
         mode: LayoutMode,
         json_schema: Option<&Value>,
     ) -> Result<Self, AlkTypeError> {
+        crate::bast_meta::validate_bast_doc(bast_doc)?;
         let doc = BastDoc::new(bast_doc, root_name)?;
         let root_def = doc.root_def();
         let struct_node = match root_def.kind() {
@@ -913,6 +914,71 @@ mod tests {
         assert!(matches!(err, AlkTypeError::Schema(_)), "got {err:?}");
     }
 
+    #[test]
+    fn compile_rejects_unknown_endian_annotation() {
+        let doc = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "endian": "middle",
+                    "fields": [ { "name": "id", "kind": "uint32" } ]
+                }
+            }
+        });
+        let err = AlkTypeEngine::compile(&doc, "S", LayoutMode::Packed, None).unwrap_err();
+        assert!(matches!(err, AlkTypeError::Schema(_)), "got {err:?}");
+    }
+
+    #[test]
+    fn compile_rejects_unknown_encoding_annotation() {
+        let doc = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "blob", "kind": "bytes", "encoding": "sideways" }
+                    ]
+                }
+            }
+        });
+        let err = AlkTypeEngine::compile(&doc, "S", LayoutMode::Packed, None).unwrap_err();
+        assert!(matches!(err, AlkTypeError::Schema(_)), "got {err:?}");
+    }
+
+    #[test]
+    fn compile_rejects_non_integer_align() {
+        let doc = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "align": "wide",
+                    "fields": [ { "name": "id", "kind": "uint32" } ]
+                }
+            }
+        });
+        let err = AlkTypeEngine::compile(&doc, "S", LayoutMode::Packed, None).unwrap_err();
+        assert!(matches!(err, AlkTypeError::Schema(_)), "got {err:?}");
+    }
+
+    #[test]
+    fn compile_accepts_inline_struct_field() {
+        let doc = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "inner", "kind": {
+                            "kind": "struct",
+                            "fields": [ { "name": "x", "kind": "uint16" } ]
+                        } }
+                    ]
+                }
+            }
+        });
+        let engine = AlkTypeEngine::compile(&doc, "S", LayoutMode::Packed, None).expect("compile");
+        assert_eq!(engine.mode(), LayoutMode::Packed);
+    }
+
     #[test]
     fn debug_formats_without_panicking() {
         let doc = fixed_struct_doc();