ant-devnet: handle SIGTERM and use a per-run data dir by default #261

Problem

ant-devnet still leaves state behind when it is stopped with SIGTERM, which is how process managers and ant dev stop stop it. This is the remainder of WithAutonomi/ant-sdk#73.

Already fixed on main: the Anvil Testnet is now kept alive until main exits (local_testnet: Option<evmlib::testnet::Testnet>) instead of std::mem::forget, so its Drop runs on a normal exit.

Still open in src/bin/ant-devnet/main.rs:

1. Only SIGINT is handled. The binary waits on tokio::signal::ctrl_c(). On SIGTERM the process dies without calling devnet.shutdown(), so cleanup_data_dir is never honoured and the Anvil child is not dropped.
2. The default data dir is shared across runs. DevnetConfig::default() uses default_root_dir() (~/.local/share/ant/ on Linux). An interrupted run leaves partial node identities there for the next run to trip over.

Proposed fix

1. Wait on SIGINT or SIGTERM (tokio::signal::unix::signal(SignalKind::terminate()) on Unix, ctrl_c elsewhere) before calling devnet.shutdown().await.
2. When --data-dir is not given, use a fresh per-run directory (tempfile::TempDir or temp_dir().join("ant_devnet_{rand}")). The manifest already records the paths, so consumers can still find them. This matches tests/e2e/testnet.rs and ant-core's MiniTestnet.

Once this lands, ant-sdk's ant dev stop band-aid (force-kill plus wiping ~/.local/share/ant/{nodes,spill}, from ant-sdk#81) can be reduced.

Background, bisect and repro: WithAutonomi/ant-sdk#73.