split-view phase 1: side-by-side listing <-> pseudocode layout

The Ghidra-style dual view, layout + toggle (no cursor sync yet — that's phase 2).
's' (and a "Split view" palette command) toggles a _split mode where the listing
(left) and pseudocode (right) show together, divided by a keyline, one focused.
Entering split loads the listing + decompiles the current function into both
panes; Tab/F5 switches the focused pane; any single-view target (hex, etc.) or 's'
again collapses back to the focused pane.

* app: _split flag; _show_active gains a split branch (both panes, load decomp,
  focus active, #panes.split class); action_toggle_split + _enter_split worker;
  action_toggle_view switches panes when split; 's' binding + palette entry;
  #panes.split ListingView divider CSS.
* docs/SPLIT_VIEW.md: the full design + phased roadmap (the hard part — line<->EA
  set mapping via a sweep of cfunc.get_line_item — is scoped for phase 3).
* tests: split_view scenario (enter/load/tab-focus/exit, 5 checks). Also fix
  disasm_nav's stale goto-bottom (press ctrl+end; plain 'end' is end-of-line now).

Verified live over RPC (both panes render, Tab flips focus, 's' exits) and pilot
split_view 5/5.

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