Share build-scoped compile-cache ownership between the launcher and runtime. Reuse inherited namespaces, avoid redundant respawns, and retire superseded builds with best-effort seven-day and 512 MiB maintenance.
Testbox: 304 focused tests passed with one Bun-only skip; pinned changed-file checks passed. Thirty child launches used 41,544 bytes instead of 1,185,000 bytes, and same-build launch time fell from 2.46 s to 1.22 s. Persistent external-plugin source capture reuse remains outside this scoped change.
