typr build --checked (soundness_transpilation.md Phase A): wraps typed
transpilation boundaries (let annotations, function params/return,
constructor calls) in typr_assert_type(...) runtime checks. A no-op
when Context::get_checked_mode() is off, or when the boundary’s type
has no reliable runtime class to check against (Any, unresolved
generics, interfaces, function types get a shallow is.function check).
Test-only oracle — never a production mode.
Render a string value as an R double-quoted literal (R’s canonical string
form). The value is assumed to be already decoded (see
parsing::elements::decode_escapes), so this is the single place that knows
how to escape for the R target: backslashes and double quotes must be
escaped, control characters are emitted as escape sequences.