deb: unmount and remove the ephemeral chroot on Ctrl-C

The SIGINT handler libc::_exit(130)s, skipping EphemeralContextGuard's
drop and leaking the freshly bootstrapped chroot with its bind-mounted
/proc and overlay mounts. Resources now register a self-contained
cleanup hook in a process-global registry that the handler drains right
before exiting: the hook unmounts everything under the chroot path
(children first, lazy fallback) and removes the tree, using only stored
paths and direct umount/rm subprocesses so it cannot deadlock on a lock
the interrupted thread may hold; sudo -n keeps it from ever hanging on
a password prompt. Drop deregisters the hook first, so the normal
cleanup path is unchanged.

Also fixes the hex grouping of the CRC-24 polynomial in apt::release.

src/ui/deb.rs

@@ -563,6 +563,22 @@ extern "C" fn on_sigint(_sig: libc::c_int) {
     {
         eprintln!("\nInterrupted — full log: {}", path.display());
     }
+    // Run the registered cleanup hooks (currently: unmount and remove the
+    // ephemeral build chroot, see `deb::ephemeral::sigint_cleanup_chroot`),
+    // then exit with the conventional 130 status.
+    //
+    // Like the terminal restoration above, this is NOT strictly
+    // async-signal-safe: it locks a mutex, spawns subprocesses and does I/O.
+    // That is a deliberate tradeoff, no worse than the rest of this handler:
+    // exiting immediately would skip all destructors and leak the chroot
+    // together with its bind-mounted /proc and overlay mounts. The hooks are
+    // self-contained (they only touch stored paths and spawn umount/rm
+    // directly), so they cannot deadlock on a lock the interrupted thread
+    // might have held; the hook registry itself is only ever taken with
+    // try_lock plus a bounded retry for the same reason. Note that SIGINT
+    // stays blocked for the duration of the handler, so a second Ctrl-C will
+    // not interrupt a slow cleanup — send SIGTERM/SIGKILL if it ever hangs.
+    crate::deb::ephemeral::run_cleanup_hooks();
     // SAFETY: raw exit bypassing destructors, intended in a signal handler
     unsafe {
         libc::_exit(130);