Output-limit spill: host-backed kernels without .in_memory() still spill to Disk

[tobert]

P3 (infra). Residuals from the SpillMode work:

• NoLocal/no-host-fs kernels are auto-forced to SpillMode::Memory at construction (kernel.rs:1144-1149), but a host-backed (localfs/Sandboxed) kernel built via Kernel::new without .in_memory() still defaults to SpillMode::Disk (output_limit.rs:77,92). None known in the wild; with_backend is the read-only path and auto-forces Memory. Note set -o output-limit[=SIZE] toggles the size at runtime (shipped), not the Disk/Memory mode.
• host's /proc, /etc reads bypass the VFS by design; gate at runtime too if "read-only" is ever marketed as "no host observation."

Migrated from docs/issues.md (P1) in the 2026-07-16 sweep; verified @ e9032af (the "no runtime switch" half of the old entry shipped as set -o output-limit).

[tobert]

Closing: the SpillMode-default half is resolved by PR #226 (Disk-by-default for host-backed kernels confirmed correct — a host-backed kernel already writes to its real mounts; both embedders use with_backend which forces Memory; the forcing matrix is now pinned by test and documented in EMBEDDING.md). The /proc//etc VFS-bypass positioning half is spun out narrow — see the issue referenced above this comment.