t27b: lower switch expressions and negative const literals like the reference #6352

Refs #6063

Problem

The native AArch64 backend cli/t27b refuses every switch expression (ExprSwitch, +4 reference-passing files in the greedy blocker list) and every folded negative literal (const X: i32 = -1;, ExprLiteral(negative literal), +4).

Plan

• Lower switch on an enum or an integer operand into a lazy right-to-left chain of selects, so only the taken prong runs (feat(t27b): lower if-expressions natively; refuse what t27c misprints #6328). The operand is evaluated once, spilled to a frame slot when it is not a plain variable.
• Lower -N literals the way t27c's Zig backend prints them: a comptime int, coerced when a type suffix is present.
• Refuse by name every shape the reference cannot compile: not exhaustive, unreachable else, duplicate prong, unknown variant, out-of-range prong, ident prong on an integer, char prong (t27c gen-zig drops needed parentheses: (if ..) + 1 and -(x - y) #6329).

Acceptance

• t27b corpus specs --blockers --reference <t27c>: mismatch stays 0; counted /720 goes up; no previously passing file regresses.
• A test in cli/t27b/tests/source.rs whose per-test verdicts equal t27c test-report.