t27b: a module var of an optional type that starts as null (agents runtime registry.t27) #7910

Part of #6063 (t27b coverage), item C10 of #6488.

What registry.t27 needs

specs/port/browseros/trios/agent-server/apps/server/src/lib/agents/runtime/registry.t27 passes the reference (9 of 9 tests). t27b refuses it first with VarDecl(module, pointer/slice):

var global_registry: ?AgentRuntimeRegistry = null;

The same refusal then cascades into ExprLiteral(null). #7470 lowered module vars that hold a string or start undefined. An optional-typed var still falls through to the pointer/slice refusal.

What the reference does

gen-zig prints the var as written: a container-level var whose initial value is null. Its payload is never read before a write. t27c test-report runs each test in a fresh process, so every test starts from null.

t27b lays out a ?T as the payload and then a flag byte, which is 0 for null. So the initial image of such a var is zero bytes, even when the payload holds a string. No address has to be written at load time, unlike #7470's string vars. Reads, writes, == null, .? and &g.? on an optional place already lower for locals.

Plan

• Accept a module var of an optional type whose initializer is null. Its image is zeros.
• Any other initial value of an optional var stays refused.
• A conformance spec: specs/tri/t27b/conformance/optional_module_var.t27. It covers a ?u32 and a ?struct holding a str, create-on-first-use through &g.?, reset to null, and a fresh null at the start of every test.

With this change, registry.t27 stops only at reg1 == reg2, which is pointer equality and is #7742's (PR #7831). With both changes it passes. Measured on the t27c lab, with that assert replaced by reg1.len == reg2.len: 9/9 under both t27b and the reference.

Acceptance criteria

• t27b test --blockers on registry.t27 names no VarDecl(module, pointer/slice) and no ExprLiteral(null).
• The conformance spec passes under t27c test-report with 0 vacuous tests, and the same tests pass under t27b.
• The lab corpus receipt compare gives IMPROVED_ONLY, EQUIVALENT or NEUTRAL.

User Scenarios

• Given a module-level variable declared with an optional type and initialized to null (e.g., var global_registry: ?AgentRuntimeRegistry = null;), when compiling with t27b, then the compiler accepts the declaration and emits a zero-initialized image for the variable without reporting VarDecl(module, pointer/slice) or ExprLiteral(null) blockers.

Requirements

• FR-001: The compiler MUST accept a module-scope variable of optional type (?T) whose initializer is exactly null and MUST lay it out as zero bytes (payload zeroed, flag byte 0).
• FR-002: The compiler MUST continue to refuse any module-scope optional variable whose initializer is not null.
• FR-003: A conformance spec at specs/tri/t27b/conformance/optional_module_var.t27 MUST be added covering ?u32 and ?struct holding a string, create-on-first-use via &g.?, reset to null, and fresh null at start of each test.
• FR-004: After the change, t27b test --blockers on specs/port/browseros/trios/agent-server/apps/server/src/lib/agents/runtime/registry.t27 MUST report zero VarDecl(module, pointer/slice) and zero ExprLiteral(null) blockers.

Boundary

• specs/port/browseros/trios/agent-server/apps/server/src/lib/agents/runtime/registry.t27
• specs/tri/t27b/conformance/optional_module_var.t27
• compiler/codegen/zig/runtime.t27