feat(benchsdk): add optional display manifest to BenchmarkConfig and upload it on upsert

[dtice25]

Summary

This is the first end-to-end slice of making a *.bench.ts file the single source of truth for both benchmark execution and how the platform displays it.

• BenchmarkConfig now accepts an optional display manifest (BenchmarkDisplayConfig) that declares:
  • metrics: key, label, unit, decimal places, and direction (lower-better | higher-better).
  • steps: human-readable labels and order for lifecycle steps.
  • overview: default ranked metric and default layout (ranking | cards | chart | leaderboard).
• defineBenchmarkConfig validates the manifest (non-empty keys/labels, valid directions, no duplicate metric keys).
• The new types are exported from @benchsdk/runner so authors get autocomplete.
• runner.ts now uploads the manifest into the platform's existing benchmarks.config JSONB field via client.upsertBenchmark(..., { config: { display: ... } }).
• benchmarks/browser/browser.bench.ts is updated with a concrete display manifest as a reference implementation.

export const config = defineBenchmarkConfig({
  // ... existing fields
  display: {
    metrics: [
      { key: 'totalMs', label: 'Total lifecycle', unit: 'ms', direction: 'lower-better', decimals: 0 },
      { key: 'createMs', label: 'Session creation', unit: 'ms', direction: 'lower-better', decimals: 0 },
    ],
    steps: [
      { key: 'create', label: 'Create session' },
      { key: 'connect', label: 'Connect CDP' },
    ],
    overview: { defaultMetric: 'totalMs', defaultLayout: 'ranking' },
  },
});

The platform counterpart (computesdk/benchmarks-platform#145) consumes this config.display to render labels, ranking direction, and default views without hard-coding the benchmark.

Link to Devin session: https://app.devin.ai/sessions/a45e3a08012c47e989ccfc4c055275ac
Requested by: @dtice25

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