t27b: lower for over a range (for (a..b) |i|, for i in a..b, for x in xs) #6349

Refs #6063

Reference-backed blocker families on master e0e806a (after #6347): StmtFor (+4 reference-passing files) and StmtFor(range) (+3), plus StmtForRange.

Scope:

• for (a..b) |i| and for i in a..b / for (i in a..=b): both bounds coerced to usize, evaluated once, length b - a checked before the first iteration (Zig panics with integer overflow when b < a), capture is a usize constant.
• for x in xs: the parser copies the loop variable into the node name; t27b read that as a labelled loop. The Zig backend ignores the name.
• Refused with a specific name: multi-object loops for (xs, ys) |x, y| as StmtFor(multi-object); comptime-reversed ranges, signed and negative bounds (compile errors in the reference).

Acceptance: per-test verdicts equal t27c test-report; mismatch 0 on t27b corpus specs --reference; a test in cli/t27b/tests/source.rs.