Function mountHexView

Mounts the virtualised hex view inside the #hex-rows container and returns a HexViewHandle for external control and cleanup.

The buffer and modifications are read from core/editor (singleton). Selection state is read from and written to core/selection (singleton).

Rendering is virtualised: only the rows within the current viewport plus an internal overscan row constant above and below are present in the DOM at any time. Top and bottom spacers simulate the full scroll height.

Features:

• Click – selects a single byte; triggers inline hex editing.
• Shift+click – extends the selection from the existing anchor.
• Drag – selects a byte range.
• Arrow keys – moves the single-byte selection one step at a time.
• Inline editing – double-click (via mouseup candidate) opens a contenteditable hex cell; Tab advances to the next byte.