Encapsulate colors.cpp's own runtime state as static, not global

[netmindz]

Summary

Part of an ongoing pass identifying WLED_GLOBAL declarations that are actually only referenced in one file (see #5777-#5786 for earlier ones). lastRandomIndex turned out to be private to colors.cpp.

Converted it to file-local static, same type and initial value as before.

No behavior change — purely a storage-class change.

Test plan

• esp32dev: builds and links cleanly via pio run -e esp32dev, no warnings.
• Confirmed via repo-wide grep (wled00/, usermods/) that lastRandomIndex is not referenced outside colors.cpp.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 0869ef4 and 6a9f8ba.

📒 Files selected for processing (2)

• wled00/colors.cpp
• wled00/wled.h

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