Greek keyboard: a word folds only when the layout types it

`greek_fold` upper-cased the word with Unicode's rules before it read the
keys. Upper-casing turns some characters the layout does not type into Greek
or ASCII letters: the micro sign into Μ, the long s into S, the sharp s into
SS, an alpha with an iota subscript into ΑΙ, the theta symbol into Θ. A stored
name with such a character then counted as a spelling of a Greek word,
although `latin_keys` does not read it.

The fold is now the word's keys with the final sigma read as the letter sigma,
so a word folds exactly when `latin_keys` reads it. Where the old fold and the
new one both exist they are the same string.

Test: five such pairs are not one word. It fails without the change.

crates/engine/src/keyboard.rs

@@ -81,9 +81,10 @@ pub fn latin_keys(word: &str) -> Option<String> {
 }
 
 /// `word` with its case, accents and final sigma folded away, when it is Greek-layout typing: every
-/// spelling of one Greek word gives the same string.
+/// spelling of one Greek word gives the same string. It is the word's keys, with the final sigma
+/// read as the letter sigma instead of the W key.
 pub fn greek_fold(word: &str) -> Option<String> {
-    latin_keys(&word.to_uppercase())
+    latin_keys(&word.replace('ς', "σ"))
 }
 
 /// Whether `a` and `b` are one Greek word, whatever their case, accents or final sigma.
@@ -116,6 +117,16 @@ mod tests {
         }
     }
 
+    /// A character the layout does not type is never taken for the Greek letter Unicode upper-cases
+    /// it to: the micro sign is not μ, the long s and the sharp s are not σ, an alpha with an iota
+    /// subscript is not αι, the theta symbol is not θ.
+    #[test]
+    fn a_character_the_layout_does_not_type_is_no_spelling_of_a_greek_letter() {
+        let pairs = [("\u{b5}", "μ"), ("Λ\u{17f}", "ΛΣ"), ("Λ\u{df}", "ΛΣΣ"), ("\u{1fb3}", "αι"), ("\u{3d1}", "θ")];
+        let same: Vec<_> = pairs.iter().filter(|(a, b)| same_greek_word(a, b) || same_greek_word(b, a)).collect();
+        assert!(same.is_empty(), "{same:?}");
+    }
+
     /// Greek-layout typing reads as its Latin keys: plain, accented and decomposed letters, the Q
     /// and W keys, ASCII prefixes.
     #[test]