fix(explorer): allow file navigation before a hidden explorer is first shown

[MarcusGasberg]

Summary

With explorer = { hidden = true }, next_file/prev_file (]f/[f) throw until the explorer has been toggled open at least once:

E5108: Lua: .../codediff/ui/explorer/actions.lua:61: Invalid 'win': Expected Lua number
stack traceback:
        [C]: in function 'nvim_win_is_valid'
        .../codediff/ui/explorer/actions.lua:61: in function 'navigate_next'
        .../codediff/ui/view/navigation.lua:210: in function <.../codediff/ui/view/navigation.lua:190>

render.create hides the split right after mounting it, and Split:hide() clears split.winid, so explorer.winid starts out nil. navigate_next/navigate_prev pass it straight to nvim_win_is_valid, which rejects nil. After the first toggle winid holds a window id (valid or stale), which is why the error only appears before the panel has ever been shown.

Changes

• lua/codediff/ui/explorer/actions.lua: guard the tree-cursor sync in navigate_next and navigate_prev with explorer.winid and ..., the same check toggle_staged_view and render.create already use.
• tests/e2e/explorer/actions_spec.lua: add [E21], which sets explorer.hidden = true, then drives next_file and prev_file from the diff pane and checks the comparison changes each time.

Benefits

explorer.hidden = true is usable with keyboard file navigation from the moment a session opens, instead of requiring a show/hide of the panel first.

Testing

• ./tests/run_tests.sh tests/e2e/explorer/actions_spec.lua: 84 passed, 0 failed (E21 passes for native/polling × side-by-side/inline). Without the fix, E21 never completes: the mapping error leaves the embedded Neovim at a hit-enter prompt and the spec hits the 300s per-spec timeout.
• ./tests/run_tests.sh (full suite, Linux, Neovim 0.12.5): 1587 passed, 18 failed. All 18 failures are in tests/integration/ui/conflict/gutter_grid_spec.lua (screen-grid glyph assertions, e.g. expected │ ╱╱╱╱, actual ╰─╱╱╱╱), and that spec fails identically on unmodified main in this environment, so it's unrelated to this change.
• stylua --check clean on both files.

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