bootstrap: v1 log tailer + exit-tail enrichment (#1369)

Summary:
Pull Request resolved: #1369

this diff finishes the parity work. earlier we dropped the auto-spawn in `ProcMeshAgent::boot_v1`. here we add proper stdout/stderr tailers on the bootstrap side, carry the stderr tail into `Stopped`, and add a targeted test that exercises it. the result is strict v0 parity: tailers wired the same way and exit tails recovered.

Reviewed By: mariusae

Differential Revision: D83504440

fbshipit-source-id: a8d8bda6fd83fbada7e47c0ecd749ed6a163876f

Author: shayne-fletcher

hyperactor_mesh/src/alloc/logtailer.rs

@@ -6,8 +6,6 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-#![allow(dead_code)] // until used
-
 use std::mem::swap;
 use std::mem::take;
 use std::ops::DerefMut;