feat(code): add parseCodeNumber helper to lib/code.ts

Pure helper that extracts the trailing numeric sequence from a code string
(ST-007 → 7, T-42 → 42). Non-conforming codes fall back to Number.MAX_SAFE_INTEGER
so they sort to the end. Includes 5 unit tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/code.ts

@@ -14,3 +14,13 @@ export function normalizeCode(input: string | null | undefined): string | null {
   const trimmed = input.trim()
   return trimmed === '' ? null : trimmed
 }
+
+/**
+ * Extract the trailing numeric sequence from a code (e.g. "ST-007" → 7, "T-42" → 42).
+ * Non-conforming codes (no trailing digits, empty string) return Number.MAX_SAFE_INTEGER
+ * so they sort to the end.
+ */
+export function parseCodeNumber(code: string): number {
+  const m = code.match(/(\d+)$/)
+  return m ? Number.parseInt(m[1], 10) : Number.MAX_SAFE_INTEGER
+}