t27b: integer constants wider than 64 bits (type u128, u256, u512, u1024), folded at compile time #7423

t27b refuses every spec that names an integer type wider than 64 bits. The corpus has five: specs/numeric/gft128.t27 (type u128), gft256.t27 (type u256), gft512.t27 (type u512), gft1024.t27 (type u1024), and specs/port/bootstrap/src/phi_f64_literals.t27 (type i128).

In the four gft specs, the wide values are module constants with literal values. t27c's Zig backend prints const X: u128 = <literal>; unchanged, so Zig's compiler folds every operation on X at compile time, at any width. Only a comparison (a bool) or an as (@intCast to a 64-bit type) ever reaches run time.

This slice adds that to t27b:

• Exact big-integer folding of wide module constants, following Zig's comptime rules: + - * must stay inside the type, << drops the bits shifted out, >> rounds toward minus infinity, / % need a non-zero divisor and non-negative operands, and two typed operands must share one type.
• A comparison of wide values becomes a constant bool.
• An as back to a standard width must fit, because @intCast at compile time is checked.
• A fn that no test reaches may name a wide type in its signature: Zig never resolves it. This works the same way as unresolved_sig does today for a struct whose layout fails.

These stay refused, by name, as type u128 and the like:

• a run-time wide value (a parameter of a reached fn, a local, a result, a wide constant mixed with a run-time operand);
• anything Zig's compiler would reject.

phi_f64_literals.t27 stays blocked. It uses i128 as a run-time value (a parameter and a local), which needs register pairs through t27b's one-register IR. It also needs tuples, str == and string indexing, which other lanes own. That is a separate slice, not this one.

Conformance spec first: specs/tri/t27b/conformance/wide_int_const.t27. t27c test-report gives 4/4 pass with 0 vacuous on the Railway lab.

Refs #6063 (lane 3). Earlier parked item: #7245.

Boundary

• specs/numeric/gft128.t27
• specs/port/bootstrap/src/phi_f64_literals.t27
• specs/tri/t27b/conformance/wide_int_const.t27