Producer B: build-time shell dev tests MISS-then-retry consistently on local runs (dev-boot prerender-phase race?) #719

Flagged during #703/#706 validation on a warm machine: producer B's 'build-time shell HIT on first request' dev tests consistently MISS on first poll and heal on retry locally, 0-flake on CI. Suspected: dev-boot prerender phase (shell-build-manifest read-through / discovery timing) racing the first request. Not user-visible (self-heals) but it weakens the first-request-HIT contract in dev and adds retry noise to every local run. Root-cause in the #701 lineage. Refs: #701, session notes 2026-07-06.