fix: replace native SVG tooltip with styled heatmap tooltip overlay

Renders an HTML tooltip below hovered diagram nodes with processor
metrics (avg, p99, % time, invocations, error rate). Styled inline
with the existing NodeToolbar pattern — positioned via screen-space
coordinates, uses DS tokens for background/border/shadow/typography.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ProcessDiagram/DiagramNode.tsx

@@ -102,18 +102,6 @@ export function DiagramNode({
       style={{ cursor: 'pointer' }}
       opacity={isSkipped ? 0.35 : undefined}
     >
-      {/* Processor metrics tooltip */}
-      {heatmapEntry && (
-        <title>{[
-          `${node.id}${heatmapEntry.processorType ? ` (${heatmapEntry.processorType})` : ''}`,
-          `Avg: ${heatmapEntry.avgDurationMs.toLocaleString(undefined, { maximumFractionDigits: 3 })}ms`,
-          `P99: ${heatmapEntry.p99DurationMs.toLocaleString(undefined, { maximumFractionDigits: 3 })}ms`,
-          `Time: ${heatmapEntry.pctOfRoute.toFixed(1)}%`,
-          heatmapEntry.totalCount != null ? `Invocations: ${heatmapEntry.totalCount.toLocaleString()}` : '',
-          heatmapEntry.errorRate != null ? `Errors: ${(heatmapEntry.errorRate * 100).toFixed(2)}%` : '',
-        ].filter(Boolean).join('\n')}</title>
-      )}
-
       {/* Selection ring */}
       {isSelected && (
         <rect