t27b lane 5: module vars that hold a string or start undefined #7448

Part of #6063 (t27b coverage, lane 5).

Two specs are refused by t27b on a module-level var that the reference runs:

• specs/boards/xc7a100t_minimal.t27: blocked on VarDecl(module, pointer/str/slice). A module var whose type holds a str (a str, a struct with a str field, an array of such structs). gen-zig prints a container-level var with a compile-time initializer, so every test starts from that value. t27b keeps module memory in a writable data image, which cannot hold a string's address before the program is loaded.
• specs/vsa/similarity_search.t27: blocked on VarDecl(module, undefined). A module var declared = undefined. Zig's Debug build, which t27c test-report uses, fills it with 0xAA bytes. This was measured with zig 0.15 on the t27b lab.

Plan:

• t27b fills an undefined module var with 0xAA bytes.
• A string-holding module var gets a prologue at the start of every test, invariant and bench that writes its initial value. This matches the reference, where each test runs in a fresh process.
• A module var of a pointer or slice type is still refused, as is a string-holding var declared undefined.
• Conformance specs are written first and each must pass t27c test-report with nonzero runtime asserts:
  • specs/tri/t27b/conformance/str_module_var.t27;
  • specs/tri/t27b/conformance/undefined_module_var.t27.

Done when:

• xc7a100t_minimal.t27 passes in t27b the way it passes in the reference.
• similarity_search.t27 moves on to its next blocker, which belongs to another lane.
• The corpus ratchet on the t27b lab shows 0 UNEXPECTED FAILURE.
• The change fits the foreign-line budget of Only-t27 gate: a foreign-line budget the label cannot lift; local hook reads exceptions from origin/master #7371.

Boundary

• specs/boards/xc7a100t_minimal.t27
• specs/tri/t27b/conformance/str_module_var.t27
• specs/tri/t27b/conformance/undefined_module_var.t27
• specs/vsa/similarity_search.t27

User Scenarios

• Given a module-level var that holds a str (or a struct/array containing a str), when the spec is compiled with t27b and run under t27c test-report, then the variable is initialized to its declared value at the start of every test, invariant, and bench, matching the reference behavior where each test runs in a fresh process.
• Given a module-level var declared = undefined, when the spec is compiled with t27b and run under t27c test-report (Zig Debug build), then the variable's memory is filled with 0xAA bytes, matching the reference behavior.

Requirements

• FR-001: t27b MUST emit a per-test prologue that writes the compile-time initializer into every module-level var whose type transitively holds a str, for all tests, invariants, and benches.
• FR-002: t27b MUST fill module-level var declared = undefined with 0xAA bytes in the writable data image.
• FR-003: t27b MUST continue to refuse module-level var of pointer or slice type, and string-holding var declared = undefined.
• FR-004: Conformance specs specs/tri/t27b/conformance/str_module_var.t27 and specs/tri/t27b/conformance/undefined_module_var.t27 MUST be added and MUST pass t27c test-report with nonzero runtime asserts.

Success Criteria

• t27c test-report specs/boards/xc7a100t_minimal.t27 exits with exit 0 and the spec passes in t27b the same way it passes in the reference.
• t27c test-report specs/vsa/similarity_search.t27 exits with exit 0 and the spec advances past the VarDecl(module, undefined) blocker (next blocker belongs to another lane).
• t27c test-report specs/tri/t27b/conformance/str_module_var.t27 exits with exit 0 and reports at least 1 runtime assert.
• t27c test-report specs/tri/t27b/conformance/undefined_module_var.t27 exits with exit 0 and reports at least 1 runtime assert.
• The t27b lab corpus ratchet shows 0 UNEXPECTED FAILURE across all specs.