feat(rsr): detect rule-table drift against the estate (#536)

The RSR rule table has rotted twice, and there is no deployed rhodibot
to catch it. Adds an offline-first drift detector, a census builder, a
pre-commit hook, and two Justfile recipes.

Shell only — Python is a banned language in this estate.

See the commit message for the parser-defensiveness and sample-census
rationale.

Author: hyperpolymath

Justfile

@@ -104,6 +104,15 @@ check-scm:
     done
     @echo "PASS: No SCM files in root"
 
+# Check the RSR rule table against the estate (drift: rules nobody can satisfy)
+validate-rsr-drift:
+    @bash bots/rhodibot/hooks/validate-rsr-drift.sh
+
+# Refresh the cached estate census that validate-rsr-drift measures against.
+# ~1 API request per repository, so it is refreshed on demand, not per commit.
+refresh-rsr-census:
+    @bash scripts/rsr-census.sh --refresh
+
 # Clean all build artifacts
 clean:
     cd robot-repo-automaton && cargo clean

bots/rhodibot/hooks/validate-rsr-drift.sh

@@ -0,0 +1,78 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: MPL-2.0
+# Copyright (c) 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+#
+# Pre-commit hook: a change to the RSR rule table is checked against the estate.
+#
+# The rule table in src/rsr.rs is the source of truth for RSR compliance, and it
+# has rotted twice before: once by demanding LICENSE.txt (0 of 269 repositories
+# carried it) and once by holding the .well-known/security.txt checks advisory
+# after the migration they were waiting on had finished.
+#
+# This hook runs scripts/detect-rsr-drift.sh whenever src/rsr.rs is part of the
+# change, so that a rule-table edit cannot land without being measured against
+# the repositories it will judge.
+#
+# The estate census is cached, not fetched here: a 269-repository crawl has no
+# business inside a commit. Refresh it first with:
+#
+#     just refresh-rsr-census
+#
+# If no census is present the hook still validates the table itself (it parses,
+# its severity tuples are well-formed, its migration posture is coherent) and
+# says loudly that the estate comparison was skipped. It does not fail the
+# commit for a missing census; it does fail it for actual drift.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../../.." && pwd)"
+DETECTOR="$REPO_ROOT/scripts/detect-rsr-drift.sh"
+RULE_TABLE="bots/rhodibot/src/rsr.rs"
+CENSUS="${RSR_CENSUS_DIR:-$REPO_ROOT/.rsr-census}"
+
+cd "$REPO_ROOT"
+
+if [ ! -x "$DETECTOR" ]; then
+    echo "ERROR: detector not found or not executable: $DETECTOR" >&2
+    exit 1
+fi
+
+# Only run when the rule table is actually part of this change.
+staged=$(git diff --cached --name-only 2>/dev/null || true)
+if ! printf '%s\n' "$staged" | grep -qxF "$RULE_TABLE"; then
+    # Also run when the detector itself is being changed, since its verdicts are
+    # only as good as its parser.
+    if ! printf '%s\n' "$staged" | grep -qxF "scripts/detect-rsr-drift.sh"; then
+        exit 0
+    fi
+fi
+
+echo "=== RSR rule-table drift check ==="
+
+if [ -d "$CENSUS" ] && [ -n "$(ls -A "$CENSUS" 2>/dev/null)" ]; then
+    repos=$(ls -1 "$CENSUS" | wc -l | tr -d ' ')
+    echo "using cached census: $CENSUS ($repos repositories)"
+    echo "  (age: $(find "$CENSUS" -maxdepth 1 -type f -printf '%TY-%Tm-%Td\n' 2>/dev/null | sort | head -1))"
+    set +e
+    "$DETECTOR" --source "$RULE_TABLE" --trees-dir "$CENSUS"
+    rc=$?
+    set -e
+else
+    echo "no census at $CENSUS — validating the table only"
+    echo "the estate comparison is SKIPPED; run: just refresh-rsr-census"
+    set +e
+    "$DETECTOR" --source "$RULE_TABLE"
+    rc=$?
+    set -e
+    # Exit 1 from a source-only run means the table itself is broken, which
+    # must block. Exit 2 means indeterminate.
+fi
+
+case "$rc" in
+    0) echo "PASS: no rule-table drift" ;;
+    1) echo "FAIL: rule-table drift found — see the report above" >&2; exit 1 ;;
+    2) echo "WARN: drift check indeterminate (some inputs unavailable)" >&2; exit 0 ;;
+    *) echo "FAIL: drift check errored (rc=$rc)" >&2; exit 1 ;;
+esac
+
+exit 0