Add narrow vs wide parquet performance benchmark tutorial (#40)

* Add narrow vs wide parquet performance benchmark tutorial

Interactive browser-based benchmarks comparing narrow (691MB, 11.6M rows)
vs wide (275MB, 2.5M rows) parquet schemas for iSamples OpenContext data.

Features:
- Three benchmarks: entity counts, site aggregation, material distribution
- Multiple runs with median timing for reliability
- Environment info display (browser, connection type)
- Data validity checks
- Technical notes on pitfalls (caching, cold starts, memory limits)

This page answers Eric's question about whether the 2-3x speedup seen locally
also holds "over the wire" with HTTP range requests via DuckDB-WASM.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* Address Codex review feedback for benchmark methodology

Fixes based on Codex code review:
- Lazy data loading: DB initialization now gated behind button click
- Sequential execution: Benchmarks wait for previous ones to complete
- Improved cold/warm methodology: Warm median excludes cold run
- Error handling: Try/catch with user-visible error display
- Pinned DuckDB version: @0.7.1 instead of @latest
- Updated methodology docs to reflect changes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* fix: Add error guards in render blocks, fix median calculation

Addresses remaining issues from Codex code review:

1. Error handling in render blocks - validityCheck and all three
   benchmarks now gracefully show error messages instead of crashing
   when they return {error}

2. Summary aggregation guards - allResults now filters out failed
   benchmarks, shows "N/A" for errors, handles null avgSpeedup

3. Documentation drift - Updated Pitfalls table to clarify we report
   warm-run median (excluding cold run)

4. warmMedian calculation - Fixed to average two elements instead of
   floor selection which overstated times

All Codex review issues now resolved.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

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Co-authored-by: Claude <[email protected]>

Author: rdhyee