Add tests for coverage gaps S1, S2, S3, S5, S6, S7 (285→346 tests, 88.9%→91.9% lines)

- S1 (error.rs): 5 tests for Display impl on all 4 AlkTypeError variants
  + Error::source(). error.rs 0%→100%.
- S2 (validation.rs, macros.rs): 28 tests for validate() (Result-returning)
  method on every validator + factory rejection arms for all 12 keywords.
  validation.rs 85%→97.5% lines / 100% fns; macros.rs 75.7%→93.1% / 100% fns.
- S3 (engine.rs): 3 tests for read_field on nested struct leaf fields, Bytes,
  and Timestamp. engine.rs fns 95%→95.3%.
- S5 (layout_builder.rs): 2 tests for union 'variant must be Struct' error
  (byte + field discriminator).
- S6 (schema.rs): 6 tests for as_str() round-trip (all 19 kinds), Display,
  needs_endian(), is_composite(), get_alktype_kind_enum(). schema.rs
  91.2%→97.5% lines / 94.3% fns.
- S7 (offset_map.rs): 1 test for nested-struct-without-properties schema error.

Updated docs/reviews/001-coverage-analysis.md with resolution section and
partial-resolved status. Remaining: S4 (sequential_reader error paths, medium
effort) and S8 (overflow guards, recommended to skip for v1).

src/offset_map.rs

@@ -870,4 +870,16 @@ mod tests {
         assert_eq!(empty.len(), 0);
         assert!(empty.is_empty());
     }
+
+    #[test]
+    fn compute_rejects_nested_struct_without_properties() {
+        let schema = json!({
+            "AlkType:Struct": true,
+            "properties": {
+                "header": { "AlkType:Struct": true }
+            }
+        });
+        let err = OffsetMap::compute(&schema).unwrap_err();
+        assert!(matches!(err, AlkTypeError::Schema(_)), "got {err:?}");
+    }
 }