feat: add shinyloadtest report subcommand

[gadenbuie]

Summary

Adds a shinyloadtest report subcommand that generates performance reports from load test output, porting the reporting features from the R shinyloadtest package.

What's included

• CLI: shinyloadtest report [dirs...] [options]
  • Auto-detects test-logs-* dirs in CWD if none specified
  • --format html|text|json (default: html)
  • Serves report on localhost by default; --output saves to file
  • --no-open to skip opening the browser
• HTML Dashboard: Self-contained interactive report with Observable Plot + D3
  • 6 sections: Sessions Gantt, Session Duration, Event Waterfall, Latency, Event Duration, Event Concurrency
  • Light/dark mode, responsive layout, URL state
  • Run selector for multi-run comparisons
• Text format: Markdown tables for terminal/CI use
• JSON format: Machine-readable output for custom analysis
• Docs: README and NEWS updated with report documentation

Architecture

• src/report/load.ts — data loading from CSV + recording
• src/report/stats.ts — server-side stat computation
• src/report/report.ts — orchestrator (serve/save)
• src/report/format-text.ts — Markdown table formatter
• src/report/format-json.ts — JSON formatter
• src/report/template.ts — HTML embedding
• src/report/app/ — Vite-built browser app

Verification

# From a directory with test output
shinyloadtest report

# Save HTML to file
shinyloadtest report --output report.html

# Markdown tables for terminal/CI
shinyloadtest report --format text

# Machine-readable JSON
shinyloadtest report --format json