feat: lock individual dividers or the whole container (#73)

[andyhorn]

Summary

Closes #73.

Adds two interaction-locking surfaces (both default true, fully back-compat):

• ResizableDivider.enabled — locks a single divider.
• ResizableContainer.resizable — locks every divider in the container at once.

A divider is interactive only when both flags are true. When locked, drag/tap/hover handlers and the resize cursor are suppressed; the divider remains visually present. Programmatic resizing via ResizableController is unaffected.

Implementation notes:

• ResizableContainerDivider takes a separate enabled parameter so the existing placeholder constructor's onResizeUpdate == null signal stays distinct from "locked."
• Effective enabled state is recomputed each build in _buildDividerSlot, so toggling either flag at runtime works without controller invalidation.
• Cursor falls back to MouseCursor.defer when locked, so any underlying cursor shows through; otherwise the configured cursor still wins.
• enabled is included in ResizableDivider.props so listEquals and the in-place update path notice config-only flips.

Test plan

• dart analyze clean
• very_good test passes
• New tests: per-divider drag suppression, per-divider tap/hover/drag callback suppression, container-wide drag suppression on multiple dividers, programmatic setSizes still works when resizable: false
• Manual sanity check in the example app

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