Workflow runtime performance visualization (heat-map overlay) #5772

Feature Summary

Texera already collects rich per-operator statistics during (and after) a workflow run: processing time, idle time, input/output row counts and byte sizes, and worker counts. Today these are only readable one operator at a time, so there is no at-a-glance way to see where the time and data actually went.

This is an umbrella issue to add a toggleable, canvas-level heat-map overlay that colors operators cold -> hot by runtime performance, turning the statistics Texera already produces into an immediate visual answer to "what is my bottleneck?". It is a read-only consumer of existing data and requires no backend changes. Follow-up from the RFC discussion in #5216.

• Toggle a "Performance" heat-map layer on the canvas, alongside the existing Grid / Regions / Workers layers
• Color every operator cold -> hot by a normalized performance score
• Switch between three views: Runtime, Throughput, and I/O imbalance
• Read an on-canvas legend showing the cold -> hot scale and the active view
• See colors update live as statistics stream in during a run
• Keep the heat-map after a page refresh, including for an already-finished run, without having to re-execute the workflow

Proposed Solution or Design

A read-only frontend feature driven by WorkflowStatusService as the single source of truth. The execution engine already streams every metric needed over the websocket, and a historical-stats API already exists for restoring a finished run after refresh, so there are no backend changes.

Heat-map Views

Key Behaviors

• Modeled on the existing toggleable canvas layers (Grid / Regions / Workers)
• Single source of truth: all metrics, live and historical, flow through WorkflowStatusService - no parallel service
• No visual conflict: the heat-map colors only the operator body fill, while the existing execution-status indicator (operator border) is left untouched
• Normalized scoring with skew handling, so one dominant operator does not flatten the rest of the scale
• Restores the last execution's statistics on load via the existing runtime-statistics API, gated on the overlay being enabled

Sub-tasks

Delivered as three sequential sub-issues, to be created and linked under this umbrella issue:

1. Capture all per-operator performance metrics in WorkflowStatusService (no UI)
2. Add the toggleable heat-map overlay layer with the three views and legend
3. Persist the toggle/view and restore the last run's statistics after refresh

Out of Scope

Rule-based hints, structural rewrite suggestions, and downloadable reports from the original RFC prototype are intentionally deferred to future discussion.

Affected Area

Workflow UI