feat(visual): optimize RxMER heat map rendering and improve MER severity color mapping

- refactor RxMER heat map from dense SVG rect rendering to canvas-based rendering for better Postman performance
- add render-bin aggregation and compact defaults to reduce horizontal overflow and DOM/render load
- use whole-number frequency labels on the heat map x-axis
- add standardized header row with optional Capture Time and optional Device Info block support
- avoid redundant MacAddress display when Device Info is present
- map heat map colors to MER severity (low MER red, mid yellow, high green)
- regenerate visual docs/previews for RxMER-Analysis-Rxmer-Heat-Map - 2026-02-23 02:46:37

Author: mgarcia01752