specs/basics/15_loops.t27: sum_to does not compile once a test calls it; the lesson passes only because nothing does #7908

Found while triaging t27b's blocked specs. Part of #6063 (t27b coverage), item C10 of #6488.

What is wrong

specs/basics/15_loops.t27 (lesson 15, upstreamed in #7799) teaches this fn:

pub fn sum_to(n: u8) -> u16 {
    var s : u16 = 0;
    for i in 0..n {
        s = s + i;
    }
    return s;
}

gen-zig prints the loop as for (0..n) |i|, so i is a usize in Zig, and s = s + i assigns a usize to a u16. Zig refuses that. The lesson's only test checks the constant SUM_BELOW_5, and no test calls sum_to. Zig analyzes a fn lazily, so it never type-checks sum_to, and the file compiles.

Evidence

The t27c lab, zig 0.16.0, t27c test-report:

• The lesson as it is: 1/1 pass.
• The same file, with assert sum_to(5) == 10; added to its test: BLOCKED, spec.zig:22:15: error: expected type 'u16', found 'usize'.
• With s = s + (i as u16); and that assert: 1/1 pass. gen-zig prints @as(u16, @intCast(i)). t27b also passes this version.

t27b (t27b test --blockers, master bbb77f0) refuses the lesson as it is, with type mismatch at line 11 (expected u16, found u64), which is the same error Zig gives. t27b is right not to pass it. The t27b ledger records the file with the reason reference-bug: the reference passes it for the wrong reason.

Proposed fix (for the course owner)

• Write the loop body as s = s + (i as u16);.
• Add a test that calls sum_to and count_up. Then the lesson's code is checked, not only its constant.

The t27b side needs no change. Once the lesson is fixed, t27b passes it as written.

User Scenarios

• Given the lesson specs/basics/15_loops.t27 compiles only because its test does not call sum_to, When a test calls sum_to(5), Then the compiler reports a type mismatch between usize and u16.
• Given the loop body is changed to s = s + (i as u16);, When the test calls sum_to(5), Then the file compiles and the assertion passes.

Requirements

• FR-001: The loop body in sum_to MUST cast the loop variable to u16 before addition.
• FR-002: The lesson's test block MUST include a call to sum_to that exercises the loop.
• FR-003: The lesson's test block MUST include a call to count_up that exercises its loop.

Success Criteria

• t27c test-report specs/basics/15_loops.t27 exits 0 and reports 2 tests passing.
• t27b test --blockers exits 0 and no longer lists specs/basics/15_loops.t27 as a reference-bug.

Boundary

• specs/basics/15_loops.t27