[bug] t27b: a local of an undeclared type in an unreached fn passes, Zig refuses it (hole in the #10897 body rule) #11053

Goal

Keep the refusal of a local whose type annotation names a type declared nowhere, in a fn no test reaches. AstGen resolves every name in a body, reached or not; since #10999 body_plan (specs/tri/t27b/lazy_sig_plan.t27) withdraws it as if it were a Sema-level refusal.

Why

Found by the TB2 lane of epic #10879 while building the constant rule (#10899): for

fn f() { var x : Missing = 0; }

with no test calling f, t27b now passes the spec and t27c test-report says BLOCKED ("use of undeclared identifier 'Missing'"). This is a false pass: t27b accepts a spec the reference refuses, and reference_disagree does not count it because the reference does not run it. The t27o corpus (1,130 of 1,216 pass) did not show it; the hole is in the rule, not in a measured spec.

Boundary

• specs/tri/t27b/lazy_sig_plan.t27
• gen/rust/tri/t27b/lazy_sig_plan.rs
• cli/t27b/tests/tail.rs
• specs/tri/t27b/conformance/lazy_signature.t27

User Scenarios

• Given a spec with an unreached fn that declares a local of an undeclared type, when t27b test runs it, then t27b refuses it, as t27c test-report does.
• Given an unreached fn whose only refusal is a Sema-level one (a unit struct returned as a value), when t27b test runs it, then it still passes, as after t27b: do not lower bodies no test reaches -- Sema-level refusals in an unreached fn are withdrawn from one plan (Closes #10897) #10999.

Requirements

• FR-001: body_plan MUST keep the refusal of a local annotated with a type declared nowhere in the file.
• FR-002: The rule MUST live in the plan spec and its tests; the hand Rust in cli/t27b MUST NOT grow (own-language net <= 0).
• FR-003: A negative control MUST show the spec above refused by t27b and BLOCKED by t27c.

Success Criteria

• t27b test specs/tri/t27b/lazy_sig_plan.t27 passes with the new tests, and cargo test --release -p t27b passes.
• The t27o corpus still passes 1,130 of 1,216 specs with 0 reference disagreements.
• The own-language check reports net <= 0 for the PR.