Add TTFB dashboard to dev endpoint

[justrach]

Feature Request

Expose TTFB (Time to First Byte) metrics in the dev server UI.

Current State

TTFB is already tracked in server.zig and logged:

info(server): GET / 266µs (ttfb: 45us)

Proposed Enhancement

Add to /_mer/debug endpoint:

• Average TTFB (last 100 requests)
• P50/P95/P99 TTFB percentiles
• Per-route TTFB breakdown
• Request duration histogram

Implementation Ideas

• Store metrics in a bounded ring buffer
• Update atomically for thread safety
• JSON endpoint for programmatic access: /_mer/debug?format=json&metrics=ttfb

Priority

P2 - Developer experience improvement