feat(wasm): add parse_month_year, base32_decode_lenient, guess_mime exports

Plan B Phase 8 — three #[wasm_bindgen] exports for the parsers migrated
in Phase 7, mirrored in extension/src/wasm.d.ts under "Pure parsers
(no session needed)". snake_case JS naming consistent with every
existing export; SessionHandle not required.

- parse_month_year(s) → { month, year } via js_value_for
- base32_decode_lenient(s) → Uint8Array
- guess_mime(filename) → string

Tests in session_tests mod cover the OK paths; error-path / JsValue
serialization can't be tested natively (JsError construction panics
off-wasm) and is covered in core (time::tests + base32::tests).

Plan C will wire SW message handlers consuming these exports in a
future round; this commit delivers only the seam.

Includes simplify-feedback fixes:
- relicario-core lib.rs module-list mentions base32 and mime
- item_types/totp.rs neighbour comment unified to ///-style block

cargo test --workspace: green
cargo clippy --workspace: silent
cargo build -p relicario-wasm --target wasm32-unknown-unknown: clean
cd extension && npm run test: 17 pre-existing failures only (baseline)

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

crates/relicario-core/src/lib.rs

@@ -14,6 +14,8 @@
 //! - [`crypto`] — Argon2id KDF (length-prefixed inputs, Zeroizing output) and
 //!   XChaCha20-Poly1305 AEAD with VERSION_BYTE 0x02.
 //! - [`ids`] — `ItemId`, `FieldId`, and content-addressed `AttachmentId`.
+//! - [`base32`] — RFC 4648 base32 codec used for TOTP secret encode/decode.
+//! - [`mime`] — Filename-extension → MIME-type guess for attachment storage.
 //! - [`time`] — unix-seconds + `MonthYear` for card expiries.
 //! - [`item_types`] — Per-type cores (`LoginCore`, `SecureNoteCore`, etc.) and the
 //!   `ItemCore`/`ItemType` enums.