perf: add -snapshots in pbench run for collecting time series metrics

[yabinma]

Add --snapshots: periodic query-JSON time-series capture

Summary

Today pbench saves a single query JSON after a query finishes — a terminal snapshot.
That's a photo; it can't answer time-based questions (when did throughput collapse,
which stage stalled, is a query in a runaway state). This PR adds an opt-in flag that
periodically captures /v1/query/{id} while the query runs, producing a time-series
of query-JSON frames — a movie instead of a photo.

What's new

• New pbench run flags (all opt-in; default off):
  • --snapshots — master switch
  • --snapshot-interval (default 30s, 5s floor)
  • --snapshot-max (default 20; 0 = unlimited)
  • --snapshot-fetch-timeout (default = interval)
• Equivalent inheritable per-stage JSON options (save_json_snapshots, …).
  Precedence: per-stage JSON > CLI flag > default.
• Frames are written verbatim (raw Presto JSON, no schema change) to a sibling
  <query>.snapshots/snapshot_<seq>_<epochMillis>.json directory — one per query.

Behavior & safety

• No regression: with --snapshots off, behavior/output is byte-identical to today
  (no .snapshots/ dir, no extra polling).
• Diagnostic mode, not for timed baseline runs (polling adds coordinator load).
• Non-overlapping polls; on stop the in-flight fetch is aborted (never delays query
  completion) and partial files are cleaned up.
• Errors never fail the query; consecutive failures back off exponentially and the
  poller is never permanently disabled. Retention keeps the earliest frame + most recent.

Testing

• Unit + function tests (mock coordinator), 93.8% coverage on the new poller, -race clean.
• Cluster-tested on a real ~10-min EXCEEDED_TIME_LIMIT query: regression pass (snapshots
  off → unchanged) and feature pass (20 frames captured, clean stop; the series shows CPU
  climbing while progress flatlines — the runaway-query signature the single terminal file misses).

Also includes a Go toolchain bump 1.26.4 → 1.26.5 to clear govulncheck GO-2026-5856
(crypto/tls); verified 0 code-affecting vulnerabilities under 1.26.5.