Render visual session dashboard in `info()` using UnicodePlots

[samtalki]

What

Enhance the info tool to optionally render a visual dashboard of session state using UnicodePlots — bar charts for variable sizes, sparklines for evaluation timing history, a summary view of all sessions.

Why

The current info() output is plain text. When working with multiple sessions, large datasets, or long-running analyses, a visual overview helps you understand what's going on at a glance — memory pressure, which session has the most state, how eval times are trending.

Ideas

Variable size bar chart

Variables in session 'default':
  df       DataFrame  ████████████████████  4.2 MB
  model    Chain      ██████████           2.1 MB
  x        Vector     ██                   0.4 MB
  result   Float64    ▏                    8 B

Multi-session overview

Sessions:
  default  ████████ worker 2  (3 vars, 6.7 MB, Revise ✓)
  test     ███      worker 3  (1 var,  0.4 MB, no Revise)
  bench    ██       worker 4  (2 vars, 0.2 MB, Revise ✓)

Eval timing sparkline

Recent eval times: ▁▂▁▁▃▇▁▁▂▁  (last 10 evals, max 2.3s)

Implementation notes

• UnicodePlots would be an optional dependency — if not installed, fall back to plain text
• The info tool already gathers variable info with types and sizes (get_worker_info)
• Could add a format="visual" parameter to info(), or auto-detect when UnicodePlots is available
• Timing history would need a small ring buffer added to SessionState