intern languageCode and pageVersion strings in Permalink (#60753)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Evan Bonsignori <evanabonsignori@gmail.com>

Author: 3 people

src/frame/lib/permalink.ts

@@ -27,6 +27,19 @@ page.permalinks is an array of objects that looks like this:
   ... and so on for each of the content file's supported versions.
 ]
 */
+// String interning pool: deduplicates low-cardinality, highly-repeated values (languageCode, pageVersion)
+// across ~975K Permalink instances (~9 languages × ~9 versions × ~65K pages). Because these fields have
+// few distinct values (~81 total), the pool stays bounded — do not use intern() for high-cardinality
+// fields like relativePath or title, where the pool would grow unbounded and leak memory.
+const stringPool = new Map<string, string>()
+
+function intern(s: string): string {
+  const pooled = stringPool.get(s)
+  if (pooled !== undefined) return pooled
+  stringPool.set(s, s)
+  return s
+}
+
 class Permalink {
   languageCode: string
   pageVersion: string
@@ -36,8 +49,8 @@ class Permalink {
   href: string
 
   constructor(languageCode: string, pageVersion: string, relativePath: string, title: string) {
-    this.languageCode = languageCode
-    this.pageVersion = pageVersion
+    this.languageCode = intern(languageCode)
+    this.pageVersion = intern(pageVersion)
     this.relativePath = relativePath
     this.title = title