[auser]

Upstream-owned continuation of fork PR #5336, preserving Casey-allard’s original implementation and test commits (original 18b58e1ad2, fork head 9105b02b42) as ancestors. The original fork cannot be pushed with the available upstream credentials. This branch merges current main without conflicts and updates the two regression test calls to the now-three-argument mailbox_grant::resolve(..., None) for requests without a workload IR.

The behavioral change remains the original narrow fix: pass a persistent launch request’s mailbox grant through canonical start admission for that invocation; ordinary later starts do not inherit it from the stored machine definition. The mock backend test must refuse unsupported mailbox access before boot, and the ordinary restart test must have no mailbox authority. No journal/wire change, mailbox-service completion, or native lifecycle claim.

Validation: cargo fmt --all -- --check, git diff --check and staged whitespace check passed. The current head d0a6543d7f8f92caed1803a64734bcb8be60b7d4 also supplies signed, host-ceiling-bounded retention in the mock request: otherwise newer production admission correctly refuses before the intended backend-capability check. Both focused cargo test --locked -p mvm-client --features test-support --lib regressions (persistent_request_mailbox_reaches_canonical_backend_admission and persistent_definition_restart_does_not_inherit_request_mailbox) passed locally; hosted PR and merge-queue checks are still required on these exact bytes. Historical Linux/native evidence belongs to #5336 at its named revision; the known original-child cleanup defect remains open. Credit and full provenance: #5336.

Do not close #5336 as superseded until this replacement is validated and queued.