DLP: the detector framework, the region-free detectors, word lists and attachment text

Phase 2a of the DLP and mail flow rules spec: pure functions in
crates/features/src/mailflow, nothing wired into the mail path yet.

- Detectors report distinct values found, each either checked by its
  published check digit or counted only beside a corroborating word
  within 50 characters. This PR adds the region-free ones: payment
  cards (issuer prefixes, Luhn), IBAN (registry lengths, mod 97),
  SWIFT/BIC, email addresses and phone numbers in bulk, dates of birth,
  passport numbers, private keys and published service-token formats.
  Regional identifiers follow, a region per PR.
- Word lists (Aho-Corasick, whole words, any case) and patterns (regex
  with a compiled-size limit) count occurrences.
- Attachment text: text files with or without a UTF-16 mark, HTML,
  DOCX/XLSX/PPTX, ODT/ODS/ODP and ZIP archives one level deep, read
  with the zip and quick-xml crates the workspace already has.
  Encrypted files, PDF, legacy binary Office files, nested archives
  and anything past the limits come back as not inspectable, with why.

21 unit tests, against the networks' test card numbers and the IBAN
registry's own examples among others.

crates/features/src/lib.rs

@@ -23,6 +23,7 @@ pub mod audit;
 pub mod branding;
 pub mod hold;
 pub mod lock;
+pub mod mailflow;
 pub mod masked_email;
 pub mod privacy;
 pub mod security;