feat(16-01): implement selectOriginalMessage three-tier attachment selection

GREEN: rfc.eml exact match -> single non-OriginatingEmail message/rfc822
candidate (covers KnowBe4 versioned filenames) -> OriginatingEmail.eml
fallback -> null. Case-insensitive on both content-type and filename
(checked via title/fullPath basename). All 8 selectOriginalMessage tests
pass; tsc clean for eml-parser files.

lib/services/eml-parser.ts

@@ -0,0 +1,62 @@
+/**
+ * EML/MIME evidence parser for the phishing-triage pipeline (Phase 16).
+ *
+ * Turns a raw RFC822/MIME `.eml` buffer (an attacker-controlled email a
+ * user reported as phishing/spam) into a normalized, structured
+ * `NormalizedMessage` — headers, structured SPF/DKIM/DMARC verdicts,
+ * URLs, and attachment metadata — using `mailparser` for MIME parsing
+ * and a small hand-rolled RFC 8601 tokenizer for Authentication-Results.
+ *
+ * Hard invariant (SC#3 / EVID-04 / T-16-03): this module must never fetch
+ * or execute anything found in a message. It never dereferences an
+ * extracted URL, never renders `mail.html`, and never performs any
+ * outbound network call while parsing. This is test-enforced with a
+ * `global.fetch` spy in eml-parser.test.ts.
+ *
+ * Also implements `selectOriginalMessage` (EVID-02) — a pure, I/O-free
+ * decision over ticket attachment metadata already in hand, choosing the
+ * originally-reported message from a ticket's attachment list.
+ */
+
+import type { Attachment } from '@/lib/types/autotask';
+
+/** Basename of an attachment's filename, lowercased, for tier matching. */
+function attachmentName(att: Attachment): string {
+  const raw = att.fullPath || att.title || '';
+  const base = raw.split('/').pop() || raw;
+  return base.toLowerCase();
+}
+
+function isMessageRfc822(att: Attachment): boolean {
+  return (att.contentType || '').toLowerCase() === 'message/rfc822';
+}
+
+/**
+ * Three-tier `.eml` attachment selection, empirically validated against 15
+ * real phishing tickets (see 16-RESEARCH.md Pitfall 1):
+ *
+ *   1. An attachment named exactly `rfc.eml` (case-insensitive) among
+ *      `message/rfc822` attachments — the Microsoft "Report Message" flow.
+ *   2. Else, among `message/rfc822` attachments, exclude any named exactly
+ *      `OriginatingEmail.eml` (case-insensitive) — if exactly one candidate
+ *      remains, select it (covers KnowBe4's versioned filenames, e.g.
+ *      `phish_alert_sp2_2.0.0.0.eml`).
+ *   3. Else (0 or 2+ ambiguous candidates after step 2) — fall back to
+ *      `OriginatingEmail.eml` if present; otherwise return null.
+ */
+export function selectOriginalMessage(attachments: Attachment[]): Attachment | null {
+  const rfc822Attachments = attachments.filter(isMessageRfc822);
+
+  const exactRfcEml = rfc822Attachments.find((att) => attachmentName(att) === 'rfc.eml');
+  if (exactRfcEml) return exactRfcEml;
+
+  const nonOriginatingCandidates = rfc822Attachments.filter(
+    (att) => attachmentName(att) !== 'originatingemail.eml'
+  );
+  if (nonOriginatingCandidates.length === 1) return nonOriginatingCandidates[0];
+
+  const originatingFallback = rfc822Attachments.find(
+    (att) => attachmentName(att) === 'originatingemail.eml'
+  );
+  return originatingFallback ?? null;
+}