fix(terminal): keep wheel scrolling inside terminal panes

[Juliusolsson05]

Problem

At a scrollback boundary, xterm leaves the wheel event unconsumed and Chromium scrolls the surrounding native panel. This makes the inline terminal jump within DebugPanel. This is separate from the GPU glyph corruption repaired in #790.

Change

Attach a disposable bubbling wheel boundary to shell, agent, and inline-debug terminal hosts. Xterm gets first refusal for normal scrollback, alternate-screen arrows, and mouse reporting. Only an otherwise-unconsumed, unmodified vertical gesture has its browser default canceled. Horizontal/modifier gestures and application engagement propagation are preserved. No synthetic PTY input, viewport movement, React updates, or repaint loop is added.

Verification

• 43 targeted tests pass across the boundary helper, all three mounted terminal lifetimes, retention, and GPU helper.
• Main/renderer type-check and test-contract checks pass after integrating current main.
• Real Electron/Chromium probe: node scripts/smoke-terminal-wheel.mjs passes; --control reproduces the original 120px parent-panel movement. Patched parent stays at 0px.
• Both modes preserve normal scrollback, the viewed line during appended output, alternate-screen arrow input, and SGR mouse reports.
• Clean diff check. Current-head CI passed (quality-gate and minimum-node-fixture-gate, run 33941434742).

Limits

This repairs the reproduced nested-scroll boundary leak; it does not claim to explain every reported scrolling symptom or change provider-specific alternate-screen behavior. The real-browser probe is opt-in because DOM test environments cannot exercise native default scrolling. The running application has not been restarted or changed by the probe.

Fixes #791