feat(whispering): add dev-only debug page for workspace metrics and stress testing

[braden-w]

Adds a debug page at /debug (dev-only, import.meta.env.DEV guard) for monitoring workspace storage and stress-testing at scale.

The page started as a comprehensive spec with browser quota monitoring, tombstone analysis, and persistent storage requests—most of which turned out to be unnecessary. Browser quota is ~77GB on a 512GB Mac (non-issue), tombstones compact to ~2 bytes each with gc: true (confirmed by existing benchmarks), and Tauri apps don't face storage eviction. What remained were the two things you can't see today: how big is the Y.Doc, and what happens when you throw thousands of rows at it.

The implementation uses two independent factory functions (createMetrics, createStressTest) that co-locate reactive $state with their operations. The component orchestrates them: { stressTest.generate(); metrics.refresh(); }. No callbacks or coupling between factories.

<!-- Template reads naturally -->
{formatBytes(metrics.current.ydocSize)}
{metrics.current.tables[0].count}

<Button onclick={() => { stressTest.generate(); metrics.refresh(); }}>
<Button onclick={() => { stressTest.deleteAll(); metrics.refresh(); }}>

What it shows:

• Y.Doc encoded size (human-readable + raw bytes)
• Per-table row counts for all 5 workspace tables
• Stress test panel: count selector (100–10K), content length selector (short/medium/long), Generate and Delete All with timing results (duration, throughput, size before/after/delta)

Changelog

• Add dev-only debug page at /debug for workspace metrics and stress testing

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