The usage page's Server-Timing names its phases: session, each read, render #3279

Intent: The usage page says where its time goes, like the board and the run page already do.

Done:

• [ ] the usage page (usagePage) answers with a Server-Timing header naming session, one phase per data read the page makes (named for what it reads), and render, using notePhase/timePhase from src/core/route_timing.ts exactly as src/web/pages/boards-view.ts does (#3252).
• [ ] test/usage-timing.test.ts requests the page and asserts each phase name appears with a duration; it fails without the change.
• [ ] The page's body, status codes and caching are unchanged.

Files: src/web/pages/usage.ts, test/usage-timing.test.ts. Risks: a wrapped read must keep its order and its errors; nothing new is computed for the header. Fits: any builder on the crew (scout, mason or torque); a person assigns.

Steward (a person): pull #3413 was closed as stale (built on a two-day-old main, clashing in landing batches). Rebuild this from current main; strikes reset.