feat(concurrency): support multi-statement fork bodies - #2923
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hir-publication-consumer count for hew-hir/src/lower.rs rises from 358 to 359: the multi-statement fork body lowering reads closure_capture_facts to materialize fork block captures, mirroring the existing closure-literal capture read.
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Closes #2912.
A
fork { ... }body was limited to a single direct call: the parser accepted full blocks, HIR rejected multi-statement and non-call bodies, and MIR lowered only the direct-call shape. A server accepting a connection could not move it into a concurrent child, forcing sequential handling.Multi-statement fork bodies now lower through the scoped-children machinery. A value created in or moved into the body is owned by the child, and its drop obligation follows the child through completion, crash, and cancellation while suspended — all three paths release exactly once, proven by the lifecycle oracles. A borrow of parent state that would dangle across the fork boundary is refused with a diagnostic naming the binding, pinned by a reject fixture.
Concurrency is proven rather than asserted: the accept fixture starts two slow children and observes both begin before either completes, under O0 and O2.
Verification
Checker, HIR task-gate, MIR cancellation-scope, codegen structural, and runtime lifecycle suites pass. The O2 differential reruns 1,328 outcomes identically. The compiled Hew ratchet passes. Suspending expressions directly inside fork-body closure shims remain outside the current codegen ABI; a coroutine-driving task wrapper is the follow-up before admitting them.