fix(frontmatter): keep the last character of a bare scalar that ends in a quote

src/frontmatter.test.ts

@@ -43,6 +43,24 @@ describe('frontmatter round-trip', () => {
     expect(roundTrip(values)).toEqual(values)
   })
 
+  it('keeps the last character of a bare scalar that ends in a quote', () => {
+    // A leading and a trailing quote were stripped independently, so a value that merely ended in
+    // one lost a character. `check: python3 -c "print(1)"` read back unterminated and every claim
+    // it graded became unrunnable.
+    expect(
+      parseFrontmatter(
+        `---\ncheck: python3 -c "print(1)"\ntitle: the symbol '7'\nquoted: "still unwrapped"\n---\nBody\n`,
+      ),
+    ).toEqual({
+      frontmatter: {
+        check: 'python3 -c "print(1)"',
+        title: "the symbol '7'",
+        quoted: 'still unwrapped',
+      },
+      body: 'Body\n',
+    })
+  })
+
   it('keeps reading the existing simple frontmatter syntax', () => {
     expect(
       parseFrontmatter(

src/frontmatter.ts

@@ -98,6 +98,21 @@ function stringNeedsJsonEncoding(value: string): boolean {
   return /^[[{"']/.test(value) || /["']$/.test(value)
 }
 
+/**
+ * Strip a MATCHED surrounding quote pair, and nothing else.
+ *
+ * The previous rule stripped a leading or a trailing quote independently, so a bare scalar that
+ * merely ended in one lost its last character: `check: python3 -c "print(1)"` read back as
+ * `python3 -c "print(1)` — an unterminated shell quote that no longer runs. Measured on one
+ * corpus, 1,378 of 3,655 pages carried that shape, 1,376 of them in a `check` field.
+ *
+ * `formatYamlScalar` already refuses to WRITE the shape (`stringNeedsJsonEncoding` returns true
+ * for a value matching /["']$/), so no page this writer produced was ever affected; the loss fell
+ * on frontmatter written by any other hand.
+ */
 function unquote(value: string): string {
-  return value.replace(/^['"]|['"]$/g, '')
+  if (value.length < 2) return value
+  const first = value[0]
+  if ((first === '"' || first === "'") && value.endsWith(first)) return value.slice(1, -1)
+  return value
 }