feat(wfe-core): add typed workflow schema system

SchemaType enum with inline syntax parsing: "string", "string?",
"list<number>", "map<string>", nested generics. WorkflowSchema
validates inputs/outputs against type declarations at both compile
time and runtime. 39 tests for parse and validate paths.

wfe-core/src/traits/mod.rs

@@ -17,4 +17,4 @@ pub use persistence::{
 pub use queue::QueueProvider;
 pub use registry::WorkflowRegistry;
 pub use search::{Page, SearchFilter, SearchIndex, WorkflowSearchResult};
-pub use step::{StepBody, StepExecutionContext, WorkflowData};
+pub use step::{HostContext, StepBody, StepExecutionContext, WorkflowData};