core_selfhost genc/switch_exhaustive.t27: build the _ => arm again, now that gen-c lowers _ as the catch-all (#7456) #7879

What

bootstrap/tests/fixtures/core_selfhost/genc/switch_exhaustive.t27 came in with #7331 (#7308). It is a gen-c repro: the core_selfhost harness builds its gen-c output with cc and runs the spec's own tests. #7331 dropped the fixture's _ => arm, the rest() fn and its two asserts, in 43dbe54. The reason was that gen-c then lowered a _ arm to (x == _), which C cannot compile.

#7456 (Closes #7428) fixed that on master: a _ switch arm is now the catch-all. So the fixture can build the arm again, and the _ catch-all gets a runnable check, not only "gen-c accepts it".

Done when

• The fixture has fn rest(x: i64) -> i64 { return switch (x) { 1 => 10, _ => 7, }; } again, with assert(rest(1) == 10) and assert(rest(5) == 7), and the header says _ counts.
• cargo test --release -p t27c --test core_selfhost passes on the Railway t27c lab.

Checked already on the t27c lab, on master ce605a6 merged with #7436: gen-c output for the restored fixture has no (x == _), and core_selfhost passes 1/1.

Part of #5980.

User Scenarios

• Given the fixture bootstrap/tests/fixtures/core_selfhost/genc/switch_exhaustive.t27 currently lacks a _ => catch-all arm in its rest function
• When the fixture is restored with fn rest(x: i64) -> i64 { return switch (x) { 1 => 10, _ => 7, }; } and assertions assert(rest(1) == 10) and assert(rest(5) == 7)
• Then the core_selfhost test passes, verifying gen-c correctly lowers _ as a catch-all in C output

Requirements

• FR-001: The fixture bootstrap/tests/fixtures/core_selfhost/genc/switch_exhaustive.t27 MUST contain the rest function with a _ => catch-all arm
• FR-002: The fixture MUST include assertions assert(rest(1) == 10) and assert(rest(5) == 7)
• FR-003: The fixture header MUST indicate that _ counts as a catch-all
• FR-004: Running cargo test --release -p t27c --test core_selfhost MUST pass (exit 0)

Boundary

• bootstrap/tests/fixtures/core_selfhost/genc/switch_exhaustive.t27