sl0p: double-click a tab to rename itHEADmain

Double-clicking a tab in the bar switches to it and drops into RenameTab mode,
matching the double-click-to-rename we added for panes in the sl0ppij zellij
fork. Enter commits, Esc cancels, exactly as the rename keybind does.

Widgets are held behind &self and cannot keep click state, so double-click
detection lives centrally in ModuleConfig::handle_mouse_action and is passed
to Widget::process_click as a flag.

THE TRAP, and the reason this is not a two-line change:
handle_mouse_action also runs for Mouse::Release. A single click therefore
arrives as LeftClick followed by Release at the SAME position. Counting
releases would make every single click register as a double click, and
resetting on release would mean a real double click never registers at all,
since the release falls between the two clicks. Only LeftClick participates;
Release neither counts nor resets.

A completed pair is consumed, so a triple click does not fire a second rename.

switch_tab_to() is called before switch_to_input_mode(), because RenameTab acts
on the ACTIVE tab - renaming without switching first would rename the wrong one.

Threshold is 400ms, the same one zellij uses for text-selection double clicks,
so the bar feels consistent with the panes.

Detection is extracted as register_click_for_double_click() so it can be tested
without constructing a ModuleConfig. Tests cover: a real double click, the
release-in-between regression, too-slow clicks, clicks at different positions,
and the triple-click case.

Upstream tests still pass: 25 total (20 upstream + 5 new).

Note: this repo's .cargo/config.toml targets wasm32-wasip1 and runs tests under
wasmtime, which is not installed here. Tests are run with
`cargo test --target x86_64-unknown-linux-gnu` instead.

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