Define invalid-tar and nonblocking-device filesystem behavior

[wdcui]

Follow-up from the static review of #1359. This work is intentionally outside the #1354-#1364 broker filesystem migration stack because both behaviors predate the stack and require broader API and compatibility decisions.

• Make tar-backed filesystem construction handle malformed archive data deliberately. litebox_broker_core/src/fs/tar_ro.rs currently treats archive bytes as trusted and uses panic-based assumptions while building TarRo. Decide on a fallible constructor contract and propagate failures consistently through broker userland and the remaining shim/runner callers.
• Define O_NONBLOCK behavior for synthetic devices. litebox_broker_core/src/fs/devices.rs currently uses unimplemented!() for nonblocking stdin, stdout, stderr, and urandom opens. Decide which devices should accept/ignore the flag, which require true nonblocking provider support, and which should return an explicit open error. Keep the current panic as an unsupported-path signal until that behavior is designed.

These items should not block or expand the current stacked PRs.