perf: scroll expanded island immediately for wheel input

[Apex-Studio-He]

Summary

• bypass AppKit's delayed elastic animation for discrete mouse-wheel events in the expanded island
• preserve native trackpad gesture and momentum handling
• clamp manual scroll positions to the document bounds
• add policy tests and document the input-routing boundary

Why

This separates the wheel-scroll responsiveness improvement from the broad Codex session changes, making the panel behavior independently reviewable.

Validation

• swiftc -parse for the changed controller and tests
• git diff --check

The local toolchain is Swift 6.1 without XCTest; CI should run OverlayPanelControllerTests before this draft is marked ready for review.

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