The keybinding parser validated strings, not whether any producer could
emit the event — so the shipped default `mouse:dblclick(sidebar-row)`
could never fire, and nothing failed.

`MouseKind::DblClick` appeared only in the enum and in parse/format;
`canonicalise` emits Down/Drag/ScrollUp/ScrollDown. Adds a `ClickTracker`
that promotes a second click at the same cell within 400ms, keeping
`canonicalise` a pure function of one event — the tracker takes `now` as a
parameter, so the timing rule is tested without sleeping.

`Region::Sidebar` and `Region::SplitDivider` are deleted: `region_at` can
only return Modal, Tab, SidebarRow, Pane or Unknown, so the parser was
accepting names the renderer cannot produce. It now rejects them.

Shift-Tab is wired to `overlays::prev_field`, which existed with no
caller, leaving the spawn modal cycleable in one direction only. crossterm
reports it as `KeyCode::BackTab` rather than a modified Tab, so `InputKey`
gains a `BackTab` variant that also forwards `\x1b[Z` to the pane.

Left alone: `Action::FocusPane(Direction)` discards its direction, but
that is v1 having one pane per tab, not a defect.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

@@ -291,0 +318,4 @@
+            row,
+        } = key
+        else {
+            return key;