Follow-up: Dhall types for the userscript submission contract #103

Deferred until the TOML submission workflow from #101 is complete. Explore a Dhall type/schema layer for the agreed userscript-submission contract, including preservation and reporting of unknown fields, offline/pinned imports, and explicit semantic checks for header agreement and hashes. This issue does not authorize changing the TOML interchange format or implementing Dhall in the current work. Coordinate the contract with hfsgtfoomw before treating it as canonical.