Make Decodex probe threads ephemeral

[yvette-carlisle]

Summary

• Mark synthetic Decodex probe thread starts as ephemeral so PROBE_OK protocol checks do not materialize on disk.
• Keep retained lane and resume paths non-ephemeral.
• Update the app-server spec and request-shape tests.

Verification

• cargo make fmt-check
• cargo nextest run -p decodex app_server::tests
• cargo make check-rust
• semantic drift helper: no stale phrase hits