systemd prototyping (#1960)

Summary:

this is research into using systemd to manage transient units and surface their output.

this change wires up an optional systemd feature in hyperactor_mesh and teaches BUCK to build with that feature. GitHub / OSS builds never enable the feature and so avoid depending on libsystemd being present on the runners.

in rust, src/systemd.rs gains more transient units tests: experiments with log aggregation.

the journald-based test is gated on `target_os = "linux"` and the `systemd` feature and then soft-fails if the journal or D-Bus session aren't available; this reflects the reality that GitHub CI and Meta devgpu/devvm journal configuration don't let us rely on that path.

in response, this diff explores a more robust approach where units write to file descriptors we pass in over D-Bus (via `UnixStream::pair` and `Fd`), and we aggregate logs from one or many units in-process using async readers.

this gives us a workable story for "systemd for unit management, Unix sockets for log transport" even where journald isn't practically usable.

Differential Revision: D87582955

Author: shayne-fletcher

hyperactor_mesh/Cargo.toml

@@ -74,6 +74,7 @@ serde_bytes = "0.11"
 serde_json = { version = "1.0.140", features = ["alloc", "float_roundtrip", "raw_value", "unbounded_depth"] }
 serde_multipart = { version = "0.0.0", path = "../serde_multipart" }
 strum = { version = "0.27.1", features = ["derive"] }
+systemd = { version = "0.10.1", optional = true }
 tempfile = "3.22"
 thiserror = "2.0.12"
 tokio = { version = "1.47.1", features = ["full", "test-util", "tracing"] }
@@ -92,5 +93,9 @@ proptest = "1.5"
 timed_test = { version = "0.0.0", path = "../timed_test" }
 tracing-test = { version = "0.2.3", features = ["no-env-filter"] }
 
+[features]
+default = []
+systemd = ["dep:systemd"]
+
 [lints]
 rust = { unexpected_cfgs = { check-cfg = ["cfg(fbcode_build)"], level = "warn" } }