refactor(auto-eval): freeze DETERMINISM_TEST_COHORT as tuple

Address review comment #3 on PR #764 Sub-project A: the determinism gate's
cohort is a fixed contract and tests shouldn't be able to mutate it.
Switching [] → () gives the CI gate the immutability the spec called for at
zero runtime cost.

Both consumers (auto_eval.compute_cqs and tests/test_determinism.py) iterate
the cohort; neither appends or indexes by slice, so the tuple is
drop-in compatible.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ychan

edgar/xbrl/standardization/tools/auto_eval.py

@@ -38,11 +38,12 @@
 
 # 10-company fixed cohort used by the determinism CI gate
 # (tests/xbrl/standardization/test_determinism.py). Sector-spread subset of
-# EXPANSION_COHORT_50 with stable baselines.
-DETERMINISM_TEST_COHORT = [
+# EXPANSION_COHORT_50 with stable baselines. Frozen as a tuple so tests can't
+# accidentally mutate the gate's cohort.
+DETERMINISM_TEST_COHORT = (
     "AAPL", "MSFT", "JPM", "BAC", "XOM",
     "WMT", "JNJ", "CAT", "V", "NEE",
-]
+)
 
 # Threshold = 5 × max(observed_noise, 0.00001). Measured 2026-04-06: observed
 # per-company EF-CQS delta was 0.0 on all 10 cohort members, so the floor wins.