research(provider): terraform_code — an HCL source provider #348

Some things are structurally absent from plan JSON: backend config, lifecycle meta-arguments
(prevent_destroy, ignore_changes, create_before_destroy), moved/import/removed blocks.
Only an HCL provider reaches them. Measured demand from public check libraries is small (a couple
dozen checks); the strategic value is running Tirith instead of a static scanner, plus in-code
suppression comments and editor tooling. Reuse conditions and eval_expression unchanged. Decide
after R3.