feat(DotsIndicator): per-dot color API for classic View indicator

[tbuonomo-agnt]

Summary

Adds first-class per-dot color support to the classic View-based DotsIndicator, addressing the long-standing request in #193.

What's new

// Each dot selected in its own accent color
dotsIndicator.selectedDotColors = intArrayOf(
    Color.parseColor("#FF5252"), // dot 0
    Color.parseColor("#FF6D00"), // dot 1
    Color.parseColor("#FFD600"), // dot 2
    Color.parseColor("#69F0AE"), // dot 3
    Color.parseColor("#40C4FF"), // dot 4
)

// Optional: custom unselected colors per dot
dotsIndicator.dotColors = intArrayOf(...)

Java-friendly setters:

dotsIndicator.setSelectedDotColors(new int[]{Color.RED, Color.GREEN, Color.BLUE});
dotsIndicator.setDotColors(new int[]{Color.LTGRAY, Color.LTGRAY, Color.LTGRAY});

API design

• selectedDotColors: IntArray? — per-index selected colors, falls back to global selectedDotColor for out-of-range or null
• dotColors: IntArray? — per-index unselected colors, falls back to global dotsColor
• resolveColor(colors, index, fallback) static helper (@JvmStatic, pure, no Android context required)
• Wired into addDot, refreshDotColor and the onPageScrolled color-interpolation path (per-index endpoints)
• 100% backward-compatible: null (default) restores current behaviour

Scope

Classic DotsIndicator (View-based) only. SpringDotsIndicator, WormDotsIndicator and Compose indicators are intentionally unchanged.

Tests

• 18 pure-JVM unit tests (DotsIndicatorColorTest) — resolveColor boundary cases, partial arrays, selected/unselected independence
• 12 Robolectric view tests (DotsIndicatorViewRenderTest) — assert actual GradientDrawable colors set on dots; covers global, per-index, partial, runtime refresh scenarios
• 4 screenshot golden scenarios for visual pin

All tests pass locally (./gradlew :viewpagerdotsindicator:testDebugUnitTest — BUILD SUCCESSFUL).

Follow-up (separate PR)

• Stabilise Roborazzi screenshot pipeline for View-based indicator goldens (dots_view_*.png)
• Consider Spring/Worm/Compose parity if demand grows

Closes #193