campaign(qwen38): the calibration floor is 12M tokens

Frank's rule: the served iq4r this arm competes with was calibrated on a
10M-token set, so the arm is calibrated at least as deeply. The floor was
lowered to 1.5M on the vendor's '10M beats 1M by under 1%' note; that
measures the vendor's own plate, not ours — the arm is judged against a
10M-calibrated reference, so cutting the calibration 6x to save wall clock
buys a worse position on the only comparison that matters.

One round of the re-chunked corpus is ~9.31M tokens, so a 12M floor seals
in round 1 plus part of round 2. A round that seals nothing is normal (the
floor is pooled) and round_verdict treats it as continue, never failure.

Also fixes tests/qwen38_campaign.bats' window dry run: its fixture lands on
/tmp (a 16 GiB tmpfs) where the 60 GiB seal preflight can never pass, so the
test now uses the documented Q38_CALIB_MIN_FREE_MIB override instead of
asserting environment luck.

tests/qwen38_campaign.bats

@@ -692,7 +692,8 @@ window_fixture() {
     D="$(window_fixture)"
     run env FRANK_QWEN38_CAMPAIGN="$D/campaign" \
         FRANK_QWEN38_RAD="$D/rad/qwen38-bf16.rad" \
-        Q38_WINDOW_CALIB_CORPUS="$D/corpora/calib.jsonl" bash "$WINDOW" --dry-run
+        Q38_WINDOW_CALIB_CORPUS="$D/corpora/calib.jsonl" \
+        Q38_CALIB_MIN_FREE_MIB=1 bash "$WINDOW" --dry-run
     [ "$status" -eq 0 ]
     [[ "$output" == *"run-qwen38-bf16.sh calib"* ]]
     [[ "$output" == *"else kill the rig"* ]]