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

mise.toml

@@ -23,3 +23,7 @@ node = "26"
 # A compile cache shared by every jj workspace's target dir: point
 # cargo's build.rustc-wrapper at it (~/.cargo/config.toml).
 sccache = "0.18.0"
+# `just package-linux` (docs/packaging.md): the AppImage and .deb by
+# cargo-packager, the .rpm by cargo-generate-rpm (D-25).
+"cargo:cargo-packager" = "0.11.8"
+"cargo:cargo-generate-rpm" = "0.21.1"