mvp v6 phase D: fake-exporter freshness probes + measure_freshness.py

[zzylol]

Summary

• Fake-exporter now emits the three v6 freshness probes
  (http_freshness_probe_{raw,warm,archive}) at 1 Hz. Each is a
  Float64Counter whose cumulative value equals the unix_ms of the
  most recent emission, so last_over_time(<probe>[10s]) returns a
  timestamp directly. Disabled with EXPORTER_FRESHNESS_PROBES=off,
  rate via EXPORTER_FRESHNESS_PROBE_HZ.
• measure_freshness.py gains a v6 poll-only mode matching the
  spec CLI (--query-endpoint / --probe / --path-label / --duration / --poll-interval-ms / --output) and v6 CSV schema
  (path,sample_ts_ms,observed_ts_ms,delta_ms). v4 mode is preserved
  for run_mvp_demo.sh on origin/main; mode is selected at runtime
  from the flag set.
• run_freshness_phase.sh drives the three probe paths sequentially
  and writes $OUT_DIR/freshness/{raw,warm,archive}.csv. Endpoints
  are overrideable via flags / ASAP_FRESHNESS_*_ENDPOINT env vars.

Routing (probe metric name → tier) is owned by the agent / gateway
pipeline configs from Phase C overlay; the fake-exporter and the
script are both path-agnostic. See
deploy/configs/mvp-v6-freshness-probes.yaml for the schema and
docs/spec-mvp-v6-controller-driven-multi-stage-demo.md §⑥ for the
protocol.

run_mvp_demo.sh and mvp_report.py are intentionally not touched —
the v5 demo is daemonized using them right now (PID 1945011 in
/tmp/asap-collector-v5-worktree). Phase E will wire the v6 demo
driver to call run_freshness_phase.sh and extend mvp_report.py
to consume the v6 CSV schema.

Test plan

• go test ./deploy/fake-exporter/... — 4 new probe tests +
  pre-existing tests all pass; verifies timestamp invariant within
  scheduler-jitter slack, EXPORTER_FRESHNESS_PROBES=off kill switch,
  monotonicity across two Collect()s, and the canonical name list
  matches the spec.
• pytest deploy/scripts/tests/ — 15 unit tests covering query
  parsing (success / empty / error / bad json / non-success), an
  end-to-end CSV write against an in-process HTTP stub, empty-result
  handling, mode-selection edge cases, and the summary formatter for
  0 / 1 / many samples.
• End-to-end smoke of run_freshness_phase.sh against a stub
  Prometheus that returns observed=now-100ms; all three CSVs land
  with delta_ms ≈ 100.
• docker build -f deploy/docker/Dockerfile.fake-exporter --build-context sketchlib-go=... -t asap/fake-exporter:dev .
  succeeds — the rebuild will be picked up next time the v6 demo
  runs.

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