/system's Server-Timing names its phases: session, each read, render #3273

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

Done:

• [ ] GET /system 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/system-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/system.ts, test/system-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.

Parked by the crew: the check failed in the place it ran, not in the change, and failed the same way when it was run again (pull #3285 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.