fix: clear eleventy-img in-memory cache between builds to prevent OOM

The MemoryCache singleton in @11ty/eleventy-img holds Sharp's processed
image ArrayBuffers indefinitely — no clear() method, no TTL, no reset.
In watch mode, external memory grows ~30-50MB per incremental rebuild
(visible as rising external/buffers in process.memoryUsage()). After
20-30 rebuilds: 500MB+ of leaked native memory, pushing RSS past the
3.5GB cgroup limit.

Fix: reset imgMemCache.cache = {} in eleventy.before hook. The disk
cache handles persistence — the memory cache is just an in-build
deduplication layer.

eleventy.config.js

@@ -380,6 +380,19 @@ export default function (eleventyConfig) {
   // page.url is unreliable during parallel rendering, but outputPath IS correct
   // since files are written to the correct location. Derives the OG slug from
   // outputPath and replaces placeholders emitted by base.njk.
+  // Clear eleventy-img in-memory cache between builds to prevent native memory leak.
+  // The MemoryCache singleton holds Sharp ArrayBuffers (~200KB-1MB each) that never get
+  // freed in watch mode. After 20-30 incremental rebuilds, external memory grows from
+  // ~170MB to 500MB+, eventually causing OOM. Disk cache handles persistence.
+  const { memCache: imgMemCache } = esmRequire("@11ty/eleventy-img/src/caches.js");
+  eleventyConfig.on("eleventy.before", () => {
+    const size = imgMemCache.size();
+    if (size > 0) {
+      imgMemCache.cache = {};
+      console.log(`[eleventy-img] Cleared in-memory cache (${size} entries)`);
+    }
+  });
+
   // Cache: directory listing built once per build instead of 3,426 existsSync calls
   let _ogFileSet = null;
   eleventyConfig.on("eleventy.before", () => { _ogFileSet = null; });