feat(ttp): E.3.8 R0041-R0048 email cohort

8 YAMLs for the email cohort per Appendix B: open-relay abuse,
mass phishing, phishing-kit X-Mailer signatures, IDN/punycode
URLs, sender masquerade, malicious attachment, BEC, encoded
payload in body. EmailLifter (E.3.12) consumes by rule_id.

test_email_rules.py: YAML-present + inert-in-v0 + xfail(strict)
precision case gated on E.3.12.

rules/ttp/R0042.yaml

@@ -0,0 +1,19 @@
+rule_id: R0042
+rule_version: 1
+name: mass_phishing_campaign
+description: |
+  RCPT count above threshold + body simhash matching across N
+  recipients in a window. EmailLifter (E.3.12).
+applies_to:
+  - email
+match:
+  kind: lifter:email_mass_phish
+  rcpt_threshold: 25
+  body_simhash_window_h: 24
+emits:
+  - tactic: TA0001
+    technique_id: T1566
+    confidence: 0.85
+evidence_fields:
+  - rcpt_count
+  - body_simhash