fix: quote YAML-special and numeric frontmatter values

formatValue now detects YAML-special tokens (true/false/null/yes/no/on/off/y/n/~)
and numeric-looking strings, quoting them via JSON.stringify so YAML parsers
cannot misread them as booleans or numbers. ISO date strings (YYYY-MM-DD) are
exempt and remain bare for readability. Adds four covering tests.

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

src/frontmatter.ts

@@ -47,6 +47,10 @@ export function serializeFrontmatter(pairs: Pair[]): string {
 
 function formatValue(v: string): string {
   if (v === "") return '""';
+  const yamlSpecial = /^(true|false|null|yes|no|on|off|y|n|~)$/i.test(v);
+  const isoDate = /^\d{4}-\d{2}-\d{2}$/.test(v);
+  const numericLike = !isoDate && /^[+-]?(\d|\.\d)/.test(v);
+  if (yamlSpecial || numericLike) return JSON.stringify(v);
   const bareSafe = /^[A-Za-z0-9_./-][A-Za-z0-9_./ -]*$/.test(v) && !/^\s|\s$/.test(v);
   return bareSafe ? v : JSON.stringify(v);
 }