TUI: recursive submodule navigation

[gfargo]

Spun out of #884 (closed). The metadata-level submodule support landed in #925 / #930; the recursive-navigation piece is large enough to track separately.

What

`Enter` on a submodule row should drill into the submodule's history, the same way `Enter` on a commit drills into its diff. Once inside, the user can navigate the submodule's commits / files / branches as if they'd run `coco ui` from inside the submodule directory. `Esc` / `<` pops back to the parent repo.

Why

Today, a user inspecting a submodule update has to:

1. See the inspector block from feat(log-tui): submodule status loader + inspector side-panel (#884) #930 (name / pinned / tracking / remote)
2. Drop to shell
3. `cd vendor/lib && coco ui`
4. Navigate to find what changed between the old and new pinned commits

The Enter-to-drill workflow collapses all four steps into one keystroke.

Implementation sketch

• New `SubmoduleStack` state on the root view model — a list of nested repo contexts. Pushing a submodule rebinds the SimpleGit instance (and every loader that consumes it) against the submodule's working dir, and pushes the prior context onto the stack. Popping restores.
• Title-bar breadcrumb so the user always knows which repo they're in (`coco-ui · vendor/lib`).
• All context-load paths must be careful about the rebind — anything cached against the parent SimpleGit instance has to be invalidated or scoped.

Effort

Large. Touches the runtime's git-instance lifecycle, every context loader, the view-stack semantics, and probably the persistence layer (tab-by-repo settings).

Related

• Submodule + git LFS awareness in the TUI #884 (umbrella, closed)
• Multi-repo workspace tracker (if/when one exists)