test(gate): make 5× the canonical gate, drop 20x naming

Rename run-20x.sh → run-gate.sh, default ARCHY_ITERATIONS 20→5, and scrub
20× references across CLAUDE.md, the master plan, TESTING.md, app-registry
status, the orchestrator/config doc-comments, and the bats suites. Also add
a minimal fail() helper to mempool.bats so guard failures report cleanly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/archipelago/src/container/prod_orchestrator.rs

@@ -894,7 +894,7 @@ pub struct ProdContainerOrchestrator {
     /// Quadlet `.container` unit and starts it via systemctl --user
     /// instead of shelling out to `podman create + start`. Default
     /// false so the legacy path remains the production path until the
-    /// 20× lifecycle harness goes green against the new path.
+    /// 5× lifecycle harness goes green against the new path.
     use_quadlet_backends: bool,
     #[cfg(test)]
     test_disk_gb: Option<u64>,
@@ -1738,7 +1738,7 @@ impl ProdContainerOrchestrator {
         } else {
             self.remove_quadlet_unit_if_present(&name).await?;
             ensure_user_podman_socket().await?;
-            // Legacy path. Production until tests/lifecycle/run-20x.sh
+            // Legacy path. Production until tests/lifecycle/run-gate.sh
             // goes green against the Quadlet path.
             self.runtime
                 .create_container(&resolved_manifest, &name, 0)