t27c lowers a Rust match block to nothing, and typecheck accepts it #5949

Defect

A Rust match block in a t27 body is parsed, typechecked and generated as nothing, and every gate stays green.

specs/auth/config.t27, get_type:

fn get_type(info: Info) -> AuthType {
    match info.type_tag {
        0 => AuthType::Oauth,
        ...
        _ => unreachable,
    }
}

• t27c gen-rust writes pub fn get_type(info: Info) -> AuthType {\n}, an empty body for a non-unit return.
• t27c gen (Zig) writes // NOT LOWERED: empty statement -- statement rendered to nothing (T43).
• t27c gen-c declares the function with no usable body.
• t27c parse and t27c typecheck both accept the spec.

Cause

W914 (bootstrap/src/compiler.rs, the match arm of the body-statement parser) captures a match block verbatim into a StmtExpr named match: "read, counted as nothing, executed as nothing". The intent was to stop a hard parse error, but it also hides the block from every check. It applies to every match block, tail or not; a match whose arms assign or return loses those effects too.

t27 already has the construct: switch (x) { a => ..., else => ... } (ExprSwitch, lowered by every backend). Rust's match with payload patterns (Ok(v) =>, Some(r) =>) has no t27 equivalent.

Plan

• typecheck refuses a captured match block, naming its line and owner, and pointing at switch. Parse still accepts it (W914's reason holds), and generation is unchanged, the same split as Fourteen specs parse, typecheck, and emit a field with no type — and a literal in type position is accepted #3225.
• Rewrite the case switch can express: auth/config get_type.
• Ledger the remaining specs, measured with t27c suite --ratchet.
• Unit tests at the end of compiler.rs.

Stacked on #5947 (#5923).

Boundary

• specs/auth/config.t27
• bootstrap/src/compiler.rs