merge: main into P11c (P11a's update/rollback, the bare-launch path, the Linux-only SOCK_CLOEXEC test)

Conflicts: ledger-cli main.rs (kept P11a's is_bare_launch; dropped the
"comes with P11" stubs), ledger-mcp local.rs (main's Linux gate on the
no-other-end test), docs/testing.md (both sections kept). `update` and
`rollback` are cfg(unix) in ledger-host (a running program's file cannot
be replaced on Windows), so main.rs says so on Windows (P11a step 5).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-host/src/lib.rs

@@ -20,6 +20,8 @@ pub mod lease;
 pub mod opener;
 mod start;
 pub mod tokens;
+#[cfg(unix)]
+pub mod update;
 
 /// How a mode stops when it can't go on: the reason is already printed,
 /// and the number is the process's exit code.