fix(inflekt): singularize coined -is/-us plurals (apis -> api)

Every word ending in -is/-us was assumed already singular, so an "apis"
table round-tripped to "apises". No suffix distinguishes the two cases
("apis" and "iris" are both a vowel plus s), so the real singulars now
come from the dictionary-generated exception table instead of a blanket
rule, and unknown words fall through to "drop the s".

Author: pyramation

packages/inflekt/__tests__/ves-and-exceptions.test.ts

@@ -139,6 +139,57 @@ describe('singularize: suffix classes the inflection library gets wrong', () =>
   });
 });
 
+describe('singularize: -is and -us words', () => {
+  it('depluralizes coined identifiers whose stem ends in a vowel', () => {
+    // The regression that started this: an "apis" table became "apises",
+    // because every -is/-us word was assumed to be already singular.
+    expect(singularize('apis')).toBe('api');
+    expect(singularize('uris')).toBe('uri');
+    expect(singularize('guis')).toBe('gui');
+    expect(singularize('cpus')).toBe('cpu');
+    expect(singularize('gpus')).toBe('gpu');
+    expect(singularizeLast('ApiSchemas')).toBe('ApiSchema');
+    expect(pluralize(singularize('apis'))).toBe('apis');
+    expect(pluralize(singularize('cpus'))).toBe('cpus');
+  });
+
+  it('leaves the dictionary singulars alone', () => {
+    // "iris" and "apis" have the same shape (a vowel plus "s"), so these are
+    // dictionary facts, not rules — see SINGULAR_EXCEPTIONS.
+    for (const word of [
+      'aegis',
+      'analysis',
+      'anxious',
+      'apparatus',
+      'bus',
+      'cactus',
+      'census',
+      'chassis',
+      'corpus',
+      'genius',
+      'iris',
+      'nucleus',
+      'radius',
+      'status',
+      'tennis',
+      'various',
+      'virus',
+    ]) {
+      expect(singularize(word)).toBe(word);
+    }
+  });
+
+  it('still depluralizes the dictionary plurals of vowel stems', () => {
+    expect(singularize('menus')).toBe('menu');
+    expect(singularize('emus')).toBe('emu');
+    expect(singularize('skis')).toBe('ski');
+    expect(singularize('taxis')).toBe('taxi');
+    expect(singularize('alibis')).toBe('alibi');
+    expect(singularize('statuses')).toBe('status');
+    expect(singularize('irises')).toBe('iris');
+  });
+});
+
 describe('SINGULAR_EXCEPTIONS table', () => {
   it('is applied for every entry', () => {
     for (const [plural, singular] of Object.entries(SINGULAR_EXCEPTIONS)) {

packages/inflekt/scripts/generate-exceptions.ts

@@ -179,6 +179,16 @@ for (const singular of [...exceptions.values()]) {
   }
 }
 
+// Round-trip closure for the singulars that end in "s" (aegis, iris, corpus):
+// their plural takes "-es", and the generic rules read that back one letter
+// short (aegises -> aegise), because "-ses" is only "-sis" for the Greek nouns.
+for (const singular of [...new Set(exceptions.values())]) {
+  if (!singular.endsWith('s')) continue;
+  const plural = pluralize(singular);
+  if (plural === singular || exceptions.has(plural)) continue;
+  if (singularizeByRules(plural) !== singular) exceptions.set(plural, singular);
+}
+
 const entries = [...exceptions.entries()].sort(([a], [b]) => a.localeCompare(b));
 const body = entries.map(([plural, singular]) => `  ${plural}: '${singular}',`).join('\n');