Draw the document request as the waterfall's first row

[nabramovitz]

Summary

The diagnostics resource waterfall never drew the document request itself: it is a navigation timing entry, not a resource entry, so the chart's first visible row was whatever loaded after the HTML arrived. On a high-latency connection that left an unexplained void covering the first second-plus of the page load.

This adds the document request as row 0 of the waterfall, segmented by phase:

• redirect, stalled, DNS, TCP, TLS, server wait, download
• Under the Stratos clock the row collapses to server wait + download — the part the app can actually influence
• The phase spans are computed once in a shared table (load-performance.ts) used by both the phase summary row and the waterfall, so the two can't disagree

Testing

• Unit specs for the new span table and the row-0 rendering (core suite green)
• Full make check gate green
• Verified live on a deployed console over a ~100ms RTT link, where the document row now accounts for the previously blank lead-in

[norman-abramovitz]

Lgtm