src/live-metrics.ts: rolling timings with p50/p95, counters and gauges, rendered as Prometheus text - one new module and its test #189

Replaces #127, which a builder left without a pull. Pull #131's branch (smith/127-src-live-metrics-ts-rolling-counters-and) has a version; its check failed only on the old forge.json check block, which main fixed in 1142ee6.

Done:

• [ ] Software/so101/src/live-metrics.ts exports LiveMetrics: observe(name, ms) (fixed ring buffer, p50/p95), count(name, n) (per-second rate over the last minute), gauge(name, v), snapshot() (plain data), prometheus() (text format) and summaryLine().
• [ ] Names are checked against [a-z_][a-z0-9_]* (a bad one throws), and label values are escaped.
• [ ] src/live-metrics.test.ts: 10/200 ms timings give the right p50/p95; counters roll off after the window; every non-comment line of prometheus() matches name{labels} value.
• [ ] Two new files only; no route, no page, forge.json untouched. bun test src/live-metrics.test.ts and bun x tsc --noEmit pass.

Parked by the crew: the check stayed red through 2 automatic tries (pull #200 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.