Accent-insensitive card search

A fold() (lowercase + strip accents, using the same table as nucleo's own
normalization) is registered as a SQLite function and used by the name,
oracle-text and location filters, so "khazad-dum" finds Troll of
Khazad-dûm. The fuzzy name search folds the query too (accented queries
used to match nothing) and its exact-name promotion; decklist/ManaBox
import falls back to a folded name match; the deck quick filter and tag
suggestions fold client-side.

Closes #17

crates/mtgc-core/src/text.rs

@@ -0,0 +1,42 @@
+//! Accent- and case-insensitive text matching.
+
+/// Lowercase `s` and strip accents ("Lim-Dûl's Cohort" → "lim-dul's cohort"),
+/// so typed text matches card names however they're accented. Uses the same
+/// per-character table as the fuzzy name search's own normalization, so both
+/// search paths agree on what counts as a match.
+pub fn fold(s: &str) -> String {
+    s.chars()
+        .map(|c| nucleo::chars::normalize(nucleo::chars::to_lower_case(c)))
+        .collect()
+}
+
+/// Register [`fold`] as the SQL function `fold(text)` on `conn`, so queries
+/// can compare `fold(oc.name)` against a folded pattern.
+pub(crate) fn register_sql_fold(conn: &rusqlite::Connection) -> rusqlite::Result<()> {
+    use rusqlite::functions::FunctionFlags;
+    conn.create_scalar_function(
+        "fold",
+        1,
+        FunctionFlags::SQLITE_UTF8 | FunctionFlags::SQLITE_DETERMINISTIC,
+        |ctx| {
+            Ok(match ctx.get::<Option<String>>(0)? {
+                Some(s) => Some(fold(&s)),
+                None => None,
+            })
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::fold;
+
+    #[test]
+    fn folds_case_and_accents_and_leaves_ascii_alone() {
+        assert_eq!(fold("Lim-Dûl's Cohort"), "lim-dul's cohort");
+        assert_eq!(fold("Éowyn, Lady of Rohan"), "eowyn, lady of rohan");
+        assert_eq!(fold("Hamato Ninpō"), "hamato ninpo");
+        assert_eq!(fold("Jakub Šlemr"), "jakub slemr");
+        assert_eq!(fold("Lightning Bolt"), "lightning bolt");
+    }
+}