t27b: untyped var from an integer literal, and an unread untyped undefined const (StmtLocal) #6995

Refs #6063 (t27b coverage lane 2, family StmtLocal).

t27b refuses two local shapes that the reference (t27c's Zig backend) compiles and runs:

1. A var set from a bare integer literal with no type, e.g. var i = 0;. t27b refuses it with "var i initialised with an untyped integer needs a type". The reference prints var i: u32 = 0;: a literal that fits u32 is typed u32, a larger one u64, and hex/binary literals are typed by value (zig_int_literal_default_type in bootstrap/src/compiler.rs). A negative or a float literal is not typed, and Zig refuses it.
2. An untyped const c = undefined; that nothing reads. t27b refuses it with "c has neither type nor value". The reference prints const c = undefined; _ = c; // dead after const-inlining, which does nothing. If the const is read, Zig refuses it, so t27b keeps refusing that case.

Files where StmtLocal is the only blocker on lab run 6600bd3:

• specs/port/trinity/src/brain/orbitofrontal_value.t27 (var i = 0; in sqrt)
• specs/port/trios/crates/trios-cli/src/cmd/submit.t27 (unused const _config = undefined;)

Plan: write the conformance spec specs/tri/t27b/conformance/untyped_local.t27 first (the reference passes 6/6, 0 vacuous), then make t27b match it in cli/t27b/src/lower.rs. The Rust edit is under the owner-approved-foreign approval on #6063.