feat(refiner): the refine loop now gates on blast radius too (G3)

[henleda]

Roadmap G3. The refiner retried until the band-aid blocked the exploit — and nothing told it
when a refinement had gone too far. A pass that widens the rule enough to catch the exploit was
accepted even if it would deny production. G2 gives it the missing half.

What changed

With a traffic sample, "works" now means two things: blocks the exploit and stays under the
blast-radius threshold.

An over-broad candidate is fed back as over_block — a diagnosis the refine agent already
understands ("blocked the LEGIT request too. Narrow the rule so it blocks only the exploit") —
with the measured rate and top blocked paths attached, so it knows what it over-blocked rather
than guessing.

Slower-but-correct, but only one round trip

Per the decision, the check runs per attempt rather than once at the end. It does not cost a
second LB: the refiner has just confirmed the candidate is enforcing, so measure_attached replays
through that LB — no second attach, no second propagation wait.

_score is now shared by simulate_policies and measure_attached, so the standalone vpcopilot simulate and the refine gate cannot drift on what "too broad" means.

No-regression

The sample comes from VPCOPILOT_SIM_LOGS or an explicit records=. With neither, the gate is
skipped and the loop behaves exactly as before — pinned by a test that raises if anything is
replayed without a sample. An unreadable sample logs a warning and refines without the gate rather
than turning a working loop into a failure.

Tests

Both directions, written before the change:

• an over-broad candidate is rejected, refined, and the agent is handed the blast radius
• a surgical policy (1 block in 200 records, 0.5%) passes on the first attempt
• an unfixable over-block gives up honestly with a reason, matching existing behaviour
• the threshold is configurable
• with no sample, nothing is replayed and the result carries no blast_radius

250 pass · coverage 73% · ruff clean.

Surfaces

No new command — the gate rides the existing refine path. VPCOPILOT_SIM_LOGS documented in
.env.example and docs/USAGE.md; the console's apply result shows the blast radius when present.

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