ShortcutScope: "hover" | "global"

Keyboard shortcuts (ARCHITECTURE.md ยง7). A small, framework-free shortcut manager for the chart: a default keymap wired to real chart actions, combos expressed as physical key codes (layout-independent), rebinding / disabling / custom commands, an alternate preset, hover-vs-global scope, and optional localStorage persistence. Pure and testable - resolve(event) and handleKey(combo) map input to a command id without needing a real DOM.