Stabilize terminal mount across pane moves and splits

[RA1NCS]

Summary

Fixes #648.

The previous fix (#896dcbf8) added snapshot save/restore to mask the symptom, but the actual root cause is that <XtermTerminal> was being unmounted and remounted whenever a buffer moved between panes (editor pane split, tab move, etc.). Snapshot replay then corrupted alt-buffer state for any TUI in flight (Claude Code, vim, htop, lazygit), produced duplicate banners, narrow reflow, and other rendering artifacts depending on timing.

This PR replaces the masking layer with a stable mount.

Architecture

• New <TerminalHost /> mounted at app root holds every live xterm instance for the lifetime of its session.
• Each pane renders a <TerminalSlot sessionId> placeholder div instead of <XtermTerminal> directly. Slots register themselves in terminal-slots-store.
• TerminalHost keeps a stable wrapper <div> per session and re-parents it (raw DOM appendChild) into whichever slot is currently registered. When no slot exists for a brief window during a pane move, the wrapper is parked in an offscreen container so xterm stays mounted.
• Buffer moves now only swap a wrapper's DOM parent — no React unmount, no PTY churn, no snapshot dance.

Other fixes folded in

• Native mousedown listener on the slot routes activation to the host pane. Portal rendering breaks React event bubbling (events bubble through React tree, not DOM tree), so the existing handlePaneMouseDownCapture in PaneContainer was never seeing clicks on the portaled xterm — pane stayed inactive, isActive stayed false, terminal looked muted (opacity-60) and the tab stayed gray.
• After in-tab split (handleSplitView), explicitly restore the initiating terminal as active. createTerminal swaps activeTerminalId to the new companion, but the companion is rendered only inside the initiator's iter in terminal-container.tsx's map — so the split layout was hidden behind the new fresh pane until the user switched tabs.

Removed

• saveSessionSnapshot / getSessionSnapshot / clearSessionSnapshot in terminal-store.
• TerminalViewSnapshot + viewSnapshot + serializedContent on Terminal.
• terminal-store-snapshot.test.ts — the behavior it covered no longer exists.
• All snapshot wiring in terminal.tsx and use-terminal-connection.ts.

Test plan

• Open a terminal, run seq 1 100, split editor pane horizontally with a markdown file → terminal pane retains scrollback, no duplicate output, no narrow reflow.
• Same with Claude Code TUI running → CC content survives split, alt-buffer not corrupted.
• Close the split → terminal reflows back to full width, no stale narrow content.
• Click into terminal pane → pane becomes active, terminal opacity normalizes, tab styling reflects active state.
• In-tab split (split icon in terminal tab bar) → both panes visible side-by-side; original session preserved.
• bun run typecheck clean.
• bun run lint clean (no new warnings).

[RA1NCS]

@mehmetozguldev Have a look please → tested, took a LONG time to fix

[mehmetozguldev]

LGTM