feat(smartem): gradient hole colouring and sequence break indicators

[vredchenko]

Summary

• Colour foilhole fills by acquisition order (blue-to-orange gradient) when the path overlay is active, giving quick-glance evaluation of ordering at the hole level
• Render dashed red line segments where the acquisition index jumps non-consecutively within a square, indicating the microscope leaves and returns later
• Show "break after" in the tooltip when hovering a hole that precedes a sequence break

Changes

AcquisitionPathOverlay.tsx

• Export lerpColor, START_COLOR, END_COLOR for reuse by SquareMap
• Add gridIndex to PathPoint interface (grid-wide acquisition index)
• Detect break segments and render them as dashed red lines across all three path styles

SquareMap.tsx

• Override foilhole fill with gradient colour when path overlay is active (0.85 opacity vs 0.6 for normal)
• Pass gridIndex through when building pathPoints
• Add hoveredBreakAfter detection and "break after" tooltip text

Test plan

• Toggle Path overlay on a square with acquisition order data
• Verify holes are coloured with blue-to-orange gradient matching their order
• Verify dashed red segments appear where acquisition index is non-consecutive
• Check all three path styles (gradient, numbered, sparse) work with gradient hole colouring
• Hover holes and verify tooltip still shows order info, plus "break after" where applicable
• Toggle Path off and verify normal metric colouring returns