Recurse into dirty submodules when building the Git status so their
modified and untracked files appear as full-path entries, and add a
commit-pointer entry (with short SHAs) only when the recorded commit
actually moved. Route diffs whose path falls inside a submodule to run
in that submodule's working tree so real per-file diffs are shown.
The changed-file list groups these under the submodule: tree view keeps
the nested structure and marks the submodule root with a badge, list
view flattens them into one expandable group pinned above the ordinary
files. Depth 1 only; ignored files are excluded; the panel stays
read-only.
Covered by client tree/list-grouping tests, parser tests, and a
server test that drives a real temporary repository and submodule.
