Cache memory-chart colors, add theme changed event

[tomk-amd]

Motivation

The memory chart would re-create a color look up structure multiple time per-frame, every time the previous C() function was called. It also would do a string look up every time to assign a color. The colors and mappings do not change unless new memory chart data is loaded. Pre-calculate all of this once, and update the colors when the theme changes.

Bonus: fix mini-map bug where colors on plot are not updated when theme changes.

Technical Details

Summary

• Add RocEvents::kThemeChanged, emitted from ApplyUserDisplaySettings when use_dark_mode flips, so widgets that cache a palette can rebuild instead of going stale.
• Stop rebuilding the memory-chart palette on every C() access. Keep a theme-owned palette on the view, classify arrows/items once into a color slot, and resolve ImU32s from that slot on layout/fetch and on theme change.
• Unlabeled arrows no longer draw "N/A"; unrecognized/empty categories use a neutral (text_main) color instead of defaulting to read.
• Rebuild the minimap event/counter bin cache on kThemeChanged so it tracks dark/light instead of staying on the startup theme.

Test plan

• Open kernel details Memory Chart: labeled arrows still show Title: value; arrows with an empty title draw the line with no caption.
• Confirm read/write/atomic/util/hit/stall colors still match the legend; unlabeled / uncategorized arrows use the neutral text color, not cyan read.
• Toggle dark/light with the memory chart visible: block, arrow, label, and legend colors update without a restart.
• Toggle dark/light with the timeline minimap visible: heatmap bins and legend update to the new theme.
• Switch kernels / wait for metric fetch: chart values update, and missing values still show N/A on titled rows (not on unlabeled arrows).