feat: headless-host metrics report

[eugypalu]

A metrics-report subcommand for truapi-host: it aggregates the metrics JSONL a
host or fleet run produces (METRICS_JSONL) into one report.

• Per category/op: count, errors, error rate, p50/p95/max latency (nearest-rank
  percentiles). Plus a per-VU rollup and a header with run id, time span, record
  count, VU count and skipped lines. Malformed lines are skipped and counted, not
  fatal; a missing file or a file with no valid records exits non-zero.
• Output: a fixed-width table by default; --json emits the structured report
  with deterministic key order (BTreeMap everywhere), so two runs diff cleanly.
• --baseline prev.jsonl: per-op deltas (Δ = current − baseline) for count,
  error rate and p50/p95, with ops marked changed / new / gone.