refactor(core): extract base32 module, dedupe two RFC 4648 impls

New crates/relicario-core/src/base32.rs hosts encode_rfc4648 +
decode_rfc4648_lenient (case-insensitive, optional padding, whitespace
stripped). Folds inline base32_encode (item.rs:255-275) and
decode_base32_totp (import_lastpass.rs:202-220) into the shared module;
both call sites updated.

- New RelicarioError::InvalidBase32(String) variant for the decoder
  error path
- Module is pub(crate); public API surface unchanged
- Steam alphabet (item_types/totp.rs:13) intentionally separate with
  neighbour comment pointing at crate::base32

Plan B Phase 7 sub-step 1 (DEV-A P2 base32 dedup half).
docs/superpowers/specs/2026-05-04-cli-restructure-design.md.

cargo test --workspace: green
cargo clippy --workspace: silent

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-core/src/base32.rs

@@ -0,0 +1,132 @@
+//! RFC 4648 base32 codec, no-padding form, lenient on input.
+//!
+//! The encoder produces canonical no-padding RFC 4648 output (uppercase ASCII).
+//! The decoder is lenient: case-insensitive, optional `=` padding, whitespace
+//! anywhere is stripped before decoding.
+//!
+//! Steam Guard's authenticator uses a different (de-ambiguated) alphabet —
+//! see `crate::item_types::totp::STEAM_ALPHABET`. That codec is intentionally
+//! NOT routed through this module.
+
+use crate::error::{RelicarioError, Result};
+
+const ALPHA: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
+
+/// RFC 4648 base32 encoder, no-padding form. Output is uppercase ASCII.
+pub fn encode_rfc4648(bytes: &[u8]) -> String {
+    let mut out = String::new();
+    let mut buffer: u32 = 0;
+    let mut bits: u32 = 0;
+    for &b in bytes {
+        buffer = (buffer << 8) | (b as u32);
+        bits += 8;
+        while bits >= 5 {
+            let idx = ((buffer >> (bits - 5)) & 0x1f) as usize;
+            out.push(ALPHA[idx] as char);
+            bits -= 5;
+        }
+    }
+    if bits > 0 {
+        let idx = ((buffer << (5 - bits)) & 0x1f) as usize;
+        out.push(ALPHA[idx] as char);
+    }
+    out
+}
+
+/// RFC 4648 base32 decoder, lenient on input.
+///
+/// Accepts upper- or lower-case letters, optional `=` padding, and whitespace
+/// anywhere. Trailing bits less than a full byte are silently discarded
+/// (canonical RFC 4648 decode).
+pub fn decode_rfc4648_lenient(s: &str) -> Result<Vec<u8>> {
+    let cleaned: String = s
+        .chars()
+        .filter(|c| !c.is_whitespace())
+        .collect::<String>()
+        .to_ascii_uppercase();
+    let trimmed = cleaned.trim_end_matches('=');
+    let mut out: Vec<u8> = Vec::with_capacity(trimmed.len() * 5 / 8);
+    let mut buffer: u32 = 0;
+    let mut bits: u32 = 0;
+    for ch in trimmed.bytes() {
+        let idx = ALPHA.iter().position(|&a| a == ch).ok_or_else(|| {
+            RelicarioError::InvalidBase32(format!("non-alphabet character {:?}", ch as char))
+        })?;
+        buffer = (buffer << 5) | (idx as u32);
+        bits += 5;
+        if bits >= 8 {
+            bits -= 8;
+            out.push(((buffer >> bits) & 0xff) as u8);
+        }
+    }
+    Ok(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn encode_rfc4648_matches_rfc_test_vectors() {
+        // RFC 4648 §10 test vectors, no-padding form.
+        assert_eq!(encode_rfc4648(b""), "");
+        assert_eq!(encode_rfc4648(b"f"), "MY");
+        assert_eq!(encode_rfc4648(b"fo"), "MZXQ");
+        assert_eq!(encode_rfc4648(b"foo"), "MZXW6");
+        assert_eq!(encode_rfc4648(b"foob"), "MZXW6YQ");
+        assert_eq!(encode_rfc4648(b"fooba"), "MZXW6YTB");
+        assert_eq!(encode_rfc4648(b"foobar"), "MZXW6YTBOI");
+    }
+
+    #[test]
+    fn decode_rfc4648_lenient_inverts_encoder_on_known_vectors() {
+        let cases: &[(&str, &[u8])] = &[
+            ("", b""),
+            ("MY", b"f"),
+            ("MZXQ", b"fo"),
+            ("MZXW6", b"foo"),
+            ("MZXW6YQ", b"foob"),
+            ("MZXW6YTB", b"fooba"),
+            ("MZXW6YTBOI", b"foobar"),
+        ];
+        for (s, want) in cases {
+            assert_eq!(&decode_rfc4648_lenient(s).unwrap()[..], *want);
+        }
+    }
+
+    #[test]
+    fn decode_rfc4648_lenient_accepts_lowercase_and_mixed_case() {
+        assert_eq!(decode_rfc4648_lenient("mzxw6").unwrap(), b"foo");
+        assert_eq!(decode_rfc4648_lenient("MzXw6yTbOi").unwrap(), b"foobar");
+    }
+
+    #[test]
+    fn decode_rfc4648_lenient_strips_optional_padding() {
+        assert_eq!(decode_rfc4648_lenient("MY======").unwrap(), b"f");
+        assert_eq!(decode_rfc4648_lenient("MZXW6===").unwrap(), b"foo");
+        assert_eq!(decode_rfc4648_lenient("MZXW6YTBOI======").unwrap(), b"foobar");
+    }
+
+    #[test]
+    fn decode_rfc4648_lenient_strips_whitespace_anywhere() {
+        assert_eq!(decode_rfc4648_lenient(" MZXW 6YTB OI ").unwrap(), b"foobar");
+        assert_eq!(decode_rfc4648_lenient("MZXW\n6YTB\tOI").unwrap(), b"foobar");
+    }
+
+    #[test]
+    fn decode_rfc4648_lenient_rejects_non_alphabet_chars() {
+        assert!(matches!(
+            decode_rfc4648_lenient("MY1"),
+            Err(RelicarioError::InvalidBase32(_))
+        ));
+        assert!(decode_rfc4648_lenient("???").is_err());
+        assert!(decode_rfc4648_lenient("MZ!XW").is_err());
+    }
+
+    #[test]
+    fn encode_decode_round_trips_arbitrary_bytes() {
+        let bytes: Vec<u8> = (0u8..=255).collect();
+        let encoded = encode_rfc4648(&bytes);
+        assert_eq!(decode_rfc4648_lenient(&encoded).unwrap(), bytes);
+    }
+}