feat(core): add BIP39 passphrase generator + zxcvbn strength gate

generate_passphrase honors word_count (3-12), separator, capitalization.
validate_passphrase_strength enforces zxcvbn score >= 3 (audit H3).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/generators.rs

@@ -1,12 +1,14 @@
 //! Password and passphrase generators. CSPRNG-only; rejection-sampled to
 //! eliminate modulo bias. Strength rating via zxcvbn.
 
+use bip39::{Language, Mnemonic};
 use rand::distributions::{Distribution, Uniform};
 use rand::rngs::OsRng;
+use rand::RngCore;
 use zeroize::Zeroizing;
 
 use crate::error::{IdfotoError, Result};
-use crate::settings::{CharClasses, GeneratorRequest, SymbolCharset};
+use crate::settings::{Capitalization, CharClasses, GeneratorRequest, SymbolCharset};
 
 const SAFE_SYMBOLS: &[u8] = b"!@#$%^&*-_=+";
 const EXTENDED_SYMBOLS: &[u8] = b"!@#$%^&*-_=+~?.";
@@ -62,6 +64,141 @@ fn random_password(
     Ok(Zeroizing::new(String::from_utf8(bytes).expect("ascii-only charset")))
 }
 
+pub fn generate_passphrase(req: &GeneratorRequest) -> Result<Zeroizing<String>> {
+    match req {
+        GeneratorRequest::Bip39 { word_count, separator, capitalization } => {
+            bip39_passphrase(*word_count, separator, *capitalization)
+        }
+        GeneratorRequest::Random { .. } => Err(IdfotoError::Format(
+            "use generate_password() for Random requests".into(),
+        )),
+    }
+}
+
+fn bip39_passphrase(word_count: u32, separator: &str, cap: Capitalization) -> Result<Zeroizing<String>> {
+    if !matches!(word_count, 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12) {
+        return Err(IdfotoError::Format("word_count must be 3..=12".into()));
+    }
+    // bip39 v2 requires entropy 128–256 bits in multiples of 32 bits (4 bytes).
+    // We always generate 128 bits (16 bytes) → 12 words, then take the first
+    // word_count words. This gives full-entropy sourcing even for short passphrases.
+    let mut entropy = Zeroizing::new([0u8; 16]);
+    OsRng.fill_bytes(entropy.as_mut_slice());
+    let m = Mnemonic::from_entropy_in(Language::English, entropy.as_slice())
+        .map_err(|e| IdfotoError::Format(format!("bip39: {e}")))?;
+    let words: Vec<String> = m.words().take(word_count as usize).map(|w| {
+        match cap {
+            Capitalization::Lower => w.to_ascii_lowercase(),
+            Capitalization::Upper => w.to_ascii_uppercase(),
+            Capitalization::FirstOfEach | Capitalization::Title => {
+                let mut chars = w.chars();
+                chars.next().map(|c| c.to_ascii_uppercase().to_string())
+                    .unwrap_or_default() + chars.as_str()
+            }
+            Capitalization::Mixed => {
+                w.chars().enumerate().map(|(i, c)| {
+                    if i % 2 == 0 { c.to_ascii_uppercase() } else { c }
+                }).collect()
+            }
+        }
+    }).collect();
+    Ok(Zeroizing::new(words.join(separator)))
+}
+
+/// Returns zxcvbn's 0-4 score (higher is stronger) and the estimated guesses.
+pub struct StrengthEstimate {
+    pub score: u8,
+    pub guesses_log10: f64,
+}
+
+pub fn rate_passphrase(p: &str) -> StrengthEstimate {
+    let est = zxcvbn::zxcvbn(p, &[]);
+    StrengthEstimate {
+        score: est.score().into(),
+        guesses_log10: est.guesses_log10(),
+    }
+}
+
+/// Strength gate at vault creation (audit H3): require score >= 3.
+pub fn validate_passphrase_strength(p: &str) -> Result<()> {
+    let est = rate_passphrase(p);
+    if est.score < 3 {
+        return Err(IdfotoError::WeakPassphrase { score: est.score });
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod bip39_tests {
+    use super::*;
+
+    #[test]
+    fn bip39_default_is_5_space_separated_words() {
+        let req = GeneratorRequest::Bip39 {
+            word_count: 5,
+            separator: " ".into(),
+            capitalization: Capitalization::Lower,
+        };
+        let pw = generate_passphrase(&req).unwrap();
+        assert_eq!(pw.split(' ').count(), 5);
+    }
+
+    #[test]
+    fn bip39_dash_separator() {
+        let req = GeneratorRequest::Bip39 {
+            word_count: 4,
+            separator: "-".into(),
+            capitalization: Capitalization::Lower,
+        };
+        let pw = generate_passphrase(&req).unwrap();
+        assert_eq!(pw.split('-').count(), 4);
+        assert!(!pw.contains(' '));
+    }
+
+    #[test]
+    fn bip39_first_of_each_capitalizes() {
+        let req = GeneratorRequest::Bip39 {
+            word_count: 5,
+            separator: " ".into(),
+            capitalization: Capitalization::FirstOfEach,
+        };
+        let pw = generate_passphrase(&req).unwrap();
+        for word in pw.split(' ') {
+            let first = word.chars().next().unwrap();
+            assert!(first.is_ascii_uppercase(), "word {word} should start uppercase");
+        }
+    }
+
+    #[test]
+    fn bip39_rejects_bad_word_count() {
+        let req = GeneratorRequest::Bip39 {
+            word_count: 2,
+            separator: " ".into(),
+            capitalization: Capitalization::Lower,
+        };
+        assert!(generate_passphrase(&req).is_err());
+    }
+
+    #[test]
+    fn rate_passphrase_strong_one_passes_gate() {
+        // 5-word bip39 passphrase
+        let req = GeneratorRequest::Bip39 {
+            word_count: 6,
+            separator: " ".into(),
+            capitalization: Capitalization::Lower,
+        };
+        let pw = generate_passphrase(&req).unwrap();
+        assert!(validate_passphrase_strength(&pw).is_ok());
+    }
+
+    #[test]
+    fn rate_passphrase_weak_fails_gate() {
+        assert!(validate_passphrase_strength("password").is_err());
+        assert!(validate_passphrase_strength("12345678").is_err());
+        assert!(validate_passphrase_strength("hunter2").is_err());
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;