refactor: extract LiveChart component, rebuild HWLiveDisplay and PerfLiveDisplay on top

[DingmaomaoBJTU]

Summary

The live terminal chart rendering is duplicated between commands/live_chart.py (perf-specific) and session/monitor/live_display.py (generic HW monitor). Refactor into a layered architecture.

Current State

• commands/live_chart.py::LiveMonitorDisplay — perf-specific, caller-driven (update per iteration), ~250 lines
• session/monitor/live_display.py::HWLiveDisplay — generic, self-driving (background thread), ~200 lines
• ~50 lines of plotext chart rendering code duplicated between both

Proposed Architecture

Layer 1: LiveChart (pure rendering component)

• Location: modelkit/utils/live_chart.py
• Responsibility: plotext chart rendering + Rich Live terminal display
• API: render_chart(time_series: dict[str, list[float]], width, height) -> Renderable
• No HW awareness — just plots named time series with colors
• Pure function, no state, no threading

Layer 2: HWLiveDisplay (HW monitor + chart)

• Location: modelkit/sysinfo/live_display.py (or session/monitor/)
• Builds on: LiveChart + HWMonitor
• Self-driving: background thread polls HWMonitor, renders via LiveChart
• API: context manager — with HWLiveDisplay(title="eval"): ...
• Used by: examples, user scripts, any non-perf context

Layer 3: PerfLiveDisplay (perf benchmark display)

• Location: modelkit/commands/live_chart.py (replaces current LiveMonitorDisplay)
• Builds on: LiveChart + HWMonitor (or inherits HWLiveDisplay)
• Caller-driven: perf loop calls update(iteration, latency_ms) per iteration
• Adds: iteration progress bar, warmup/benchmark phase, latency/throughput status
• Used by: wmk perf --monitor

Benefits

• Eliminates ~50 lines of duplicated plotext chart code
• Clear separation: rendering vs monitoring vs benchmarking
• LiveChart reusable for future visualizations (e.g., training loss, memory over time)
• Each layer has a single responsibility

Files to Modify

File	Action
modelkit/utils/live_chart.py	Create — pure chart renderer
modelkit/sysinfo/live_display.py	Create — HWLiveDisplay (move from session/monitor/)
modelkit/commands/live_chart.py	Rewrite — PerfLiveDisplay using LiveChart
modelkit/session/monitor/live_display.py	Delete — replaced by sysinfo/

Acceptance Criteria

• LiveChart renders arbitrary time series without HW knowledge
• HWLiveDisplay works as a standalone context manager (current behavior preserved)
• PerfLiveDisplay renders iteration progress + HW chart (current wmk perf --monitor behavior preserved)
• No plotext rendering code duplicated across files
• examples/imgcls_eval.py continues to work with --monitor
• wmk perf --monitor continues to work
• All existing tests pass