[raiseCatError]

Stacked on #495 (chain: dev ← #492 ← #493 ← #494 ← #495 ← this; #489–#491 are already in dev).

• New split mode: the editor (only editable pane) beside NMSh's own preview of the SAME buffer. F2 (new splitToggle key) or s from the read-only views; Ctrl+O switches the focused pane; keys go to exactly one pane.
• Preview follows the caret via the source map (rowForLine); l unlinks; x swaps sides; < > width 30–70%; preview pane has its own find (/) and heading prompt (#).
• Below 100 columns only the focused pane is drawn full width; widening restores the split. Snippets, list/table helpers, recovery, hunk review, Save As (Playground) all work in the source pane.
• Not in this PR (ledger): pointer input in the split, vertical orientation, Chromium preview inside the split, preview→caret sync.

Tests: documentSplit.test.ts (11, incl. a real-app test, hostile and 40k-line documents, NO_COLOR). Combined run with viewer/workspace/editor/input-ownership suites: 145 pass. Not run: full suite, physical QA.
Refs #461.