feat: Notepad++-style split view (two editor panes)

[codecancu]

Enable View → Split Horizontal / Split Vertical (previously disabled). Faithful two-view model: two independent editor panes, each with its own tab strip; documents belong to one pane (move-only, no live clone); the split persists across reload.

• DocumentStore: per-doc view tag, per-view active tracking + focused view, listForView/activeForView/moveToView/hasView; focused-view-aware active()/activeId/setActive shims keep existing callers working.
• DockManager: secondary editor group (addSecondaryEditorGroup / removeSecondaryEditorGroup / isSplit / onEditorFocusChange), two-phase host→mount so the CM6 view is created only after the dock panel exists.
• editor-page: second EditorView/EditorController via a lazy factory; shared extensions parameterised per pane; Lua bridge + inspectors follow the focused pane.
• App: view/controller getters over focused refs route every command to the focused pane with no per-call-site churn; split orchestration, Move to Other View, collapse-on-empty, session restore of the split.
• TabBar: view-scoped rendering + Move to Other View context item.
• StatusBar: cursor follows the focused pane (document-level, view-tagged).
• Persistence: SessionSnapshot gains per-view active ids + split orientation (additive; no IndexedDB version bump).

Tests: +5 store multi-view, +5 dock split-group unit tests; new split-view.spec.ts e2e (create / independent edit / move + collapse / reload persistence). 640 unit + 179 e2e pass; typecheck, lint, build clean.

What & why

Checklist

• Signed off (DCO -s flag on all commits)
• npm run lint passes
• npm run typecheck passes
• npm test passes
• Added or updated tests for behavior changes
• New source files include // SPDX-License-Identifier: GPL-3.0-or-later