Introduce FormValue as the parser-availability bound for #[slhx::form] models, blanket it for FromStr types, and add compile-fail coverage for a form field whose domain type lacks a parser.
req: form/004
req: form/006
Add #[slhx::form] for user-authored form models, emit form field metadata into slhx.syms, and require form handlers to accept models that passed generated field compatibility checks.
req: form/001
req: form/004
req: form/006
req: codegen/004