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/mailflow/detectors/mod.rs

@@ -0,0 +1,199 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! Detectors (dlp-and-mail-flow-rules spec, §2.3): each finds one kind of
+//! identifier in text and reports the distinct ones it found.
+//!
+//! A detector is one of two strengths:
+//!
+//! - **Checked**: the identifier carries a published check digit or
+//!   checksum, so a random number rarely passes; found on its own.
+//! - **Needs a word**: the format alone is too common, so a candidate counts
+//!   only with a corroborating word within [`WINDOW`] characters either
+//!   side.
+//!
+//! Findings are distinct normalized values (digits only, upper case), so the
+//! same card number pasted twice counts once. They stay in memory: callers
+//! read only [`Findings::len`].
+
+pub mod any;
+pub mod checks;
+
+use ahash::AHashSet;
+
+/// How far, in characters, a corroborating word may be from a candidate.
+pub const WINDOW: usize = 50;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Strength {
+    Checked,
+    NeedsWord,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Region {
+    Any,
+    Us,
+    Uk,
+    Canada,
+    Australia,
+    Eu,
+    Europe,
+    Asia,
+    Americas,
+    Africa,
+}
+
+/// The distinct values one detector found.
+#[derive(Debug, Default)]
+pub struct Findings(AHashSet<String>);
+
+impl Findings {
+    pub fn insert(&mut self, value: impl Into<String>) {
+        self.0.insert(value.into());
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+pub struct Detector {
+    /// Stable id, stored in rules: `payment-card`, `iban`, `us-ssn`.
+    pub id: &'static str,
+    pub name: &'static str,
+    pub region: Region,
+    pub strength: Strength,
+    find: fn(&str, &mut Findings),
+}
+
+impl Detector {
+    pub const fn new(
+        id: &'static str,
+        name: &'static str,
+        region: Region,
+        strength: Strength,
+        find: fn(&str, &mut Findings),
+    ) -> Self {
+        Self {
+            id,
+            name,
+            region,
+            strength,
+            find,
+        }
+    }
+
+    /// Adds what this detector finds in `text` to `findings`. Call once per
+    /// piece of text (subject, each part, each attachment) with the same
+    /// `findings`, then read its length.
+    pub fn find(&self, text: &str, findings: &mut Findings) {
+        (self.find)(text, findings)
+    }
+
+    /// The distinct values found in one text.
+    pub fn count(&self, text: &str) -> usize {
+        let mut findings = Findings::default();
+        self.find(text, &mut findings);
+        findings.len()
+    }
+}
+
+/// Every detector, in the order the console lists them.
+pub fn all() -> impl Iterator<Item = &'static Detector> {
+    any::DETECTORS.iter()
+}
+
+pub fn by_id(id: &str) -> Option<&'static Detector> {
+    all().find(|detector| detector.id == id)
+}
+
+/// Whether one of `words` appears, as a whole word and ignoring case, within
+/// [`WINDOW`] characters before `start` or after `end` (byte offsets of the
+/// candidate in `text`). The window is widened by the longest word, so a
+/// word that reaches into it still counts whole.
+pub fn word_near(text: &str, start: usize, end: usize, words: &[&str]) -> bool {
+    let reach = WINDOW + words.iter().map(|w| w.chars().count()).max().unwrap_or(0);
+    let before = text[..start]
+        .char_indices()
+        .rev()
+        .nth(reach - 1)
+        .map_or(0, |(i, _)| i);
+    let after = text[end..]
+        .char_indices()
+        .nth(reach)
+        .map_or(text.len(), |(i, _)| end + i);
+    let window = text[before..after].to_lowercase();
+    words.iter().any(|word| contains_word(&window, word))
+}
+
+/// Whether `word` (lower case) appears in `haystack` (lower case) with no
+/// letter or digit on either side.
+pub fn contains_word(haystack: &str, word: &str) -> bool {
+    haystack.match_indices(word).any(|(i, _)| {
+        let before_ok = haystack[..i]
+            .chars()
+            .next_back()
+            .is_none_or(|c| !c.is_alphanumeric());
+        let after_ok = haystack[i + word.len()..]
+            .chars()
+            .next()
+            .is_none_or(|c| !c.is_alphanumeric());
+        before_ok && after_ok
+    })
+}
+
+/// Whether the match at `start..end` stands alone: no digit or letter
+/// directly before or after it, so `123-45-6789` isn't found inside a
+/// longer run of digits.
+pub fn stands_alone(text: &str, start: usize, end: usize) -> bool {
+    let before = text[..start].chars().next_back();
+    let after = text[end..].chars().next();
+    before.is_none_or(|c| !c.is_alphanumeric()) && after.is_none_or(|c| !c.is_alphanumeric())
+}
+
+/// The ASCII digits of `s`.
+pub fn digits(s: &str) -> String {
+    s.chars().filter(char::is_ascii_digit).collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn words_are_whole_and_near() {
+        let text = "Your passport number is X1234567, thanks";
+        let start = text.find("X123").unwrap();
+        assert!(word_near(text, start, start + 8, &["passport"]));
+        assert!(!word_near(text, start, start + 8, &["pass"]));
+        let far = format!("passport{}X1234567", " ".repeat(60));
+        let start = far.find("X123").unwrap();
+        assert!(!word_near(&far, start, start + 8, &["passport"]));
+    }
+
+    #[test]
+    fn near_counts_characters_not_bytes() {
+        // 45 two-byte characters between the word and the candidate: within
+        // 50 characters, though over 50 bytes
+        let text = format!("passport {} X1234567", "é".repeat(45));
+        let start = text.find("X123").unwrap();
+        assert!(word_near(&text, start, start + 8, &["passport"]));
+    }
+
+    #[test]
+    fn ids_are_unique() {
+        let mut seen = AHashSet::new();
+        for detector in all() {
+            assert!(seen.insert(detector.id), "duplicate id {}", detector.id);
+            assert!(by_id(detector.id).is_some());
+        }
+    }
+}