Fix flatten_spectra_multi colors and trange handling

[huangzesen]

Summary

• Preserve caller-provided colors when flatten_spectra_multi is used with explicit time= values.
• Implement trange= handling by setting the averaging range and using the range midpoint as the representative selected time.
• Update the COLORS and TRANGE header comments to match the actual selected-time/spectrum behavior.

Details

• Avoid KEYWORD_SET(colors) for default-color detection because color index 0 is valid.
• Generate default colors only after selected_times is known.
• If both time= and trange= are supplied, this implementation uses trange= and documents that precedence.

Validation

• git diff --check
• Static checks for trange_in handling, late default colors, time= preserving caller colors, existing averaging through trange, and final end
• Zhipu/GLM review: no blockers; requested TRANGE/TIME precedence doc note applied
• IDL runtime was not available on my PATH, so no local IDL smoke run was performed

Closes #52.
Closes #53.