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/R0043.yaml

@@ -0,0 +1,22 @@
+rule_id: R0043
+rule_version: 1
+name: phishing_kit_xmailer
+description: |
+  X-Mailer header matches a curated phishing-kit signature DB
+  (PHPMailer-of-known-kits, GreatHorn known-bad, etc.).
+applies_to:
+  - email
+match:
+  kind: lifter:email_xmailer_kit
+  catalog: phishing_kits
+emits:
+  - tactic: TA0001
+    technique_id: T1566
+    confidence: 0.9
+  - tactic: TA0042
+    technique_id: T1588
+    sub_technique_id: T1588.001
+    confidence: 0.85
+evidence_fields:
+  - x_mailer
+  - matched_kit