fix(quant): a flat calibration round is not a failure — the tap's floor is pooled

RADIANCE_CALIB_TOKENS=12000000 is a pooled floor and calib-7168.jsonl carries 10.03M tokens, so
round 1 of a corpus push seals nothing by construction. The gate PR #84 added failed exactly
there, one round too early, while the drain was already running.

Now round_verdict() separates the cases: 'shrink' (the tap lost bytes) fails the run, 'flat' says
so and continues, 'grew' continues, 'done' stops. A run that never sees the engine's completion
line still exits non-zero on max rounds, and the rig marker remains the lock that stops a push
into anything that is not the rig.

Test: round_verdict's four verdicts, behaviorally.

tests/qwen38_campaign.bats

@@ -699,3 +699,20 @@ window_fixture() {
     [[ "$output" == *"calib-push.py"* ]]
     [[ "$output" == *"AGENTTRASH"* ]]
 }
+
+@test "a flat calibration round is not a failure: the tap's floor is pooled" {
+    # round 1 of a 10.03M-token corpus against a 12M floor seals nothing, and the
+    # first version of this gate aborted the run there — one round too early.
+    run python3 - "$PUSHER" <<'PY'
+import importlib.util, sys
+spec = importlib.util.spec_from_file_location("cp", sys.argv[1])
+m = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(m)
+print(m.round_verdict(8192, 8192, False),   # flat: continue
+      m.round_verdict(8192, 4096, False),   # shrink: fail
+      m.round_verdict(8192, 20000, False),  # grew: continue
+      m.round_verdict(0, 0, True))          # done: stop
+PY
+    [ "$status" -eq 0 ]
+    [ "$output" = "flat shrink grew done" ]
+}