feat: add slashing penalty core

docs/slashing-penalty-demo.md

@@ -0,0 +1,51 @@
+# Slashing Penalty Core Demo
+
+This snippet shows the focused slashing penalty core applying double-vote
+evidence to a validator balance and future epoch enrollment rows.
+
+```bash
+PYTHONPATH=node python - <<'PY'
+import sqlite3
+from slashing_penalties import apply_slashing_evidence, filter_slashed_validators
+
+db = sqlite3.connect(":memory:")
+db.executescript("""
+CREATE TABLE balances (miner_id TEXT PRIMARY KEY, amount_i64 INTEGER NOT NULL);
+CREATE TABLE epoch_enroll (
+    epoch INTEGER NOT NULL,
+    miner_pk TEXT NOT NULL,
+    weight REAL NOT NULL,
+    PRIMARY KEY (epoch, miner_pk)
+);
+INSERT INTO balances VALUES ('validator-a', 1000000);
+INSERT INTO epoch_enroll VALUES (10, 'validator-a', 1.0);
+INSERT INTO epoch_enroll VALUES (11, 'validator-a', 1.0);
+INSERT INTO epoch_enroll VALUES (12, 'validator-a', 1.0);
+""")
+
+result = apply_slashing_evidence(
+    db,
+    {
+        "validator_id": "validator-a",
+        "offense_type": "double_vote",
+        "epoch": 10,
+        "details": {"vote_a": "root-a", "vote_b": "root-b"},
+    },
+    current_epoch=10,
+    slash_fraction=0.10,
+    exclusion_epochs=2,
+    now_ts=1234,
+)
+print(result["penalty_urtc"], result["slashed_until_epoch"], result["removed_future_enrollments"])
+print(db.execute("SELECT amount_i64 FROM balances WHERE miner_id='validator-a'").fetchone()[0])
+print(filter_slashed_validators(db, ["validator-a", "validator-b"], 11))
+PY
+```
+
+Expected output:
+
+```text
+100000 12 2
+900000
+['validator-b']
+```