test(reader-arc): pin deterministic tune/measure split BEFORE grammar work

AMENDMENT 3 (PR #76): the reader grammar family is developed against the TUNE
split only; increment deltas are measured on the disjoint MEASURE split only.
Split = pure function of case id (measure iff sha256(id) even, else tune), so
disjointness is enforced by construction and cannot drift. Fixed 2026-07-18,
never re-seeded. Sizes: tune=261, measure=239. This commit lands BEFORE any
grammar-tuning commit — the git history is the proof the split was pinned in
advance (criterion 3). wrong=0 is verified across the full 500 regardless of
split; only the coverage delta is attributed to the measure split.

evals/gsm8k_math/holdout_dev/v1/split.py

@@ -0,0 +1,25 @@
+"""Deterministic tune / measure split of the official holdout_dev/v1 (ADR-0250 reader arc).
+
+Pinned **before any grammar work** (compare increment plan §5, AMENDMENT 3): the reader grammar
+family is developed against the **tune** split only; every increment's coverage delta is measured on
+the disjoint **measure** split only. The assignment is a pure function of the case id, so it is
+reproducible and cannot drift — disjointness is enforced by construction, not by discipline.
+
+Rule (fixed 2026-07-18, never re-seeded): a case is ``measure`` iff ``sha256(id)`` is even, else
+``tune`` (≈ 50 / 50). `wrong=0` is verified across the **full 500** regardless of split; only the
+*coverage delta* is attributed to the measure split.
+"""
+from __future__ import annotations
+
+import hashlib
+
+__all__ = ["split_of", "TUNE", "MEASURE"]
+
+TUNE = "tune"
+MEASURE = "measure"
+
+
+def split_of(case_id: str) -> str:
+    """Return ``"tune"`` or ``"measure"`` for ``case_id`` — deterministic, id-only."""
+    digest = int(hashlib.sha256(case_id.encode("utf-8")).hexdigest(), 16)
+    return MEASURE if digest % 2 == 0 else TUNE