[Code scan] Scatter plot reports latency under the efficiency field

This issue was found by a Codex global repository scan of tracked non-test files at commit `8c93925cb10b401b2b83c738bd9263fd74474468`.

### Relevant code
https://github.com/deepmodeling/LAMBench/blob/8c93925cb10b401b2b83c738bd9263fd74474468/lambench/metrics/vishelper/plot_generation.py#L35-L49
https://github.com/deepmodeling/LAMBench/blob/8c93925cb10b401b2b83c738bd9263fd74474468/lambench/metrics/vishelper/metrics_calculations.py#L200-L216
https://github.com/deepmodeling/LAMBench/blob/8c93925cb10b401b2b83c738bd9263fd74474468/lambench/metrics/results/README.md#L109-L111

### Impact
The ranking code defines efficiency as `100 / average_time`, and the README describes the same inverse-latency score. `generate_scatter_plot()` stores `efficiency_raw["average_time"]` directly under the JSON field named `efficiency`.

This makes the scatter data use the opposite direction from the ranking score: larger values mean slower models in the scatter JSON but better models in the ranking metric.

### Suggested fix
Either emit `100 / average_time` for the scatter `efficiency` field, or rename the field to `latency` and update downstream visualization labels and docs accordingly.