validate_json against consumer-provided JSON Schema (step 6)

- AlkTypeEngine::compile gains a 4th param json_schema: Option<&Value>.
  When Some, a standard jsonschema::Validator is built from the
  consumer-provided JSON Schema and stored for the JSON-validation
  path. When None, validate_json returns AlkTypeError::Schema and
  is_valid_json returns false (D-BAST-007).

- validate_json / is_valid_json signatures unchanged (per semver
  contract). Behavior: they now validate against the consumer JSON
  Schema, not a custom-keyword validator built from the alktype
  schema. The BAST document is not involved in this path.

- build_validator repurposed (deferred decision #2): same signature,
  now builds a standard jsonschema::Validator with no custom keywords.
  Behavioral break, not a type break. Re-export kept.

- Removed the 19 jsonschema::Keyword implementations and the 4
  define_*_validator! macros (dead on the bytes path since step 5,
  now dead on the JSON path too). The is_rfc3339_timestamp helper
  lives on in bast_validation.rs (already copied there in step 5).

- All compile call sites updated to pass None for json_schema (the
  layout/read/write/validate_bytes tests don't need JSON validation).

- New tests: validate_json accepts/rejects against consumer JSON
  Schema, returns Schema error when no JSON Schema supplied,
  is_valid_json false when no schema, independence from BAST doc,
  malformed JSON Schema build error, nested object JSON Schema.

Verification:
- cargo test --release: 409 pass (332 lib + 77 integration)
- cargo clippy --all-targets -- -D warnings: clean
- cargo build --target wasm32-unknown-unknown --release: clean
- cargo doc --no-deps: clean

tests/poc_roundtrip.rs

@@ -87,7 +87,7 @@ fn fixed_size_round_trip_via_engine_aligned() -> Result<(), AlkTypeError> {
             }
         }
     });
-    let engine = AlkTypeEngine::compile(&doc, "S", LayoutMode::Aligned)?;
+    let engine = AlkTypeEngine::compile(&doc, "S", LayoutMode::Aligned, None)?;
     let offset_map = engine.offset_map().expect("aligned mode has offset_map");
     let mut buffer = vec![0u8; offset_map.total_size()];
 
@@ -129,7 +129,7 @@ fn string_round_trip_via_engine_aligned() -> Result<(), AlkTypeError> {
             }
         }
     });
-    let engine = AlkTypeEngine::compile(&doc, "S", LayoutMode::Aligned)?;
+    let engine = AlkTypeEngine::compile(&doc, "S", LayoutMode::Aligned, None)?;
     let offset_map = engine.offset_map().expect("aligned mode has offset_map");
     let mut buffer = vec![0u8; offset_map.total_size() + 64];
     engine.write_field(&mut buffer, "name", &FieldValue::String("hello"))?;
@@ -254,7 +254,7 @@ fn nested_struct_round_trip_via_engine_aligned() -> Result<(), AlkTypeError> {
             }
         }
     });
-    let engine = AlkTypeEngine::compile(&doc, "S", LayoutMode::Aligned)?;
+    let engine = AlkTypeEngine::compile(&doc, "S", LayoutMode::Aligned, None)?;
     let offset_map = engine.offset_map().expect("aligned mode");
 
     assert_eq!(offset_map.get("header.version").unwrap().start, 0);