Gate: the newest period's reach rows must be what rule_reach.csv says now

[mmcky]

Closes #21 — the reach half; the score half landed in #23.

rule_reach_history.csv is written by qestyle_scan.py --append-history, an optional flag, and the file is committed. So a scan run without it does not leave the file absent — it leaves the previous run's rows present under the current period's label. rule_reach.csv moves, the history does not, and everything that reads the history (the trend chart, check_narrative, the front page's trend sentence) is happy with the stale row.

What changes

New check_reach_history in the gate. Both files are written from the same counts, so the newest period's history rows must equal rule_reach.csv per rule — lectures_affected and total_occurrences, rule sets equal in both directions — its corpus_size must be snapshot.json's n_lectures, and share_pct must be reach / corpus_size. Green on main today (35 of 35 rules agree). Runbook and pass-measure now say the gate enforces --append-history instead of asking people to remember it.

Verified

mutation	result
unmodified tree	pass
one occurrence count changed in the history	FAIL, names the rule and both values
a rule missing from the history	FAIL — "--append-history was not run"
a rule missing from rule_reach.csv	FAIL — "the history is from another run"
corpus_size ≠ n_lectures	FAIL
rule_reach.csv absent	FAIL, no traceback
the real #21 shape: scan re-run, history not	FAIL

Not in this PR

Making --period / --append-history mandatory in the scan (the prevention half) touches qestyle_scan.py, which moves the checker digest and forces a re-measure of both periods. It is bundled with the other two scan-side changes in #24, to be taken with the next detector change. Checker digest unchanged here (aef064f3b260).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new consistency-gate check to prevent publishing a stale rule_reach_history.csv “latest period” row when a scan is run without --append-history, and updates the runbook/skill docs to reflect that the gate now enforces this invariant (reach half of #21).

Changes:

• Add check_reach_history to tools/qestyle_check.py to hold the newest period’s history rows to rule_reach.csv and snapshot.json.
• Update UPDATE.md and pass-measure skill text to document that --append-history is effectively mandatory because the gate now fails on stale reach history rows.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tools/qestyle_check.py	Introduces the new reach-history gate check and wires it into main().
UPDATE.md	Updates the runbook narrative + adds a new “Reach history” row to the consistency checks table.
.claude/skills/pass-measure/SKILL.md	Aligns the measurement skill instructions with the new enforced reach-history invariant.

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