fix(ci): measure route bundles from webpack manifests

The performance report measured nothing. It only read `entryJSFiles` from
each route's client-reference manifest, a field Turbopack emits and webpack
does not. When the build moved to webpack (3d828a61) every route fell
through to the "unavailable" branch, and because the report is informational
and exits 0 on an unavailable metric, nothing failed and the budgets quietly
stopped being enforced.

Derive the envelope from clientModules[*].chunks when entryJSFiles is
absent, which is the same source Next's own static-routes-info uses for
webpack builds. Webpack interleaves numeric chunk ids with file names in
those arrays, so ids are skipped by shape while a malformed chunk path still
throws — otherwise a broken manifest would quietly under-report a route.
entryJSFiles still wins when present, since it is per-segment and therefore
the tighter envelope, and the per-chunk origin label is shared rather than
the absolute node_modules path webpack records, which would otherwise bloat
report.json.

Six tests cover the webpack layout: id filtering, deduplication of a chunk
reached by several client modules, the origin label, the malformed-path
rejection, the no-chunks-at-all case, and entryJSFiles taking precedence.

Re-measured on the current build, all six routes are inside their budgets
again. Note /admin/studio/furni now sits at ~98% of its gzip limit, so one
more dependency on that route will trip it; docs/performance-budgets.md
records the webpack baseline numbers and how to recalibrate.

Verified: 3385 tests, typecheck and biome clean, and the report now emits
measured rows instead of six unavailable ones.

scripts/performance-report.mjs

@@ -79,6 +79,27 @@ function filesFrom(values) {
 	return values.map(normalizeAsset);
 }
 
+/**
+ * Webpack interleaves numeric chunk ids with file names in `chunks` arrays, so
+ * a real file has to be separated from its id. An id is rejected by
+ * normalizeAsset for the right reason (it has no `static/` prefix and no `.js`
+ * suffix), which makes it a usable filter — but only for ids. A malformed
+ * *path* must still fail loudly rather than be silently dropped, or a broken
+ * manifest would quietly under-report a route's real weight.
+ */
+function assetFilesFromChunkList(values) {
+	if (!Array.isArray(values))
+		throw new Error("Unsupported JavaScript chunk list.");
+	const files = [];
+	for (const value of values) {
+		if (typeof value !== "string")
+			throw new Error("Non-string JavaScript asset.");
+		if (/^\d+$/.test(value)) continue;
+		files.push(normalizeAsset(value));
+	}
+	return files;
+}
+
 export function measureRoute({
 	budget,
 	appPath,
@@ -86,18 +107,44 @@ export function measureRoute({
 	clientManifest,
 	readAsset,
 }) {
+	// Turbopack emits an explicit per-segment `entryJSFiles` list. Webpack does
+	// not — it only records chunks per client module — so after the build moved
+	// to webpack (3d828a61) every route reported "unavailable" and the report
+	// silently stopped measuring anything. Fall back to the same source Next's
+	// own `static-routes-info` uses for webpack builds.
 	const entries = clientManifest?.entryJSFiles;
-	if (!entries || typeof entries !== "object" || Array.isArray(entries))
+	const webpackModules = clientManifest?.clientModules;
+	let filesBySource;
+	let webpackLayout = false;
+	if (entries && typeof entries === "object" && !Array.isArray(entries)) {
+		const sourceEntries = Object.keys(entries);
+		if (
+			!sourceEntries.some((key) =>
+				key.replaceAll("\\", "/").endsWith(`/app${appPath}`),
+			)
+		)
+			throw new Error("Route page entry is absent from entryJSFiles.");
+		filesBySource = Object.entries(entries);
+	} else if (webpackModules && typeof webpackModules === "object") {
+		webpackLayout = true;
+		// Each `chunks` array is `[chunkId, fileName, chunkId, fileName, ...]`.
+		filesBySource = [];
+		for (const node of Object.values(webpackModules)) {
+			if (!Array.isArray(node?.chunks) || node.chunks.length === 0) continue;
+			// One shared origin label instead of the module path: the per-chunk
+			// `sources` list is written into report.json, and webpack records
+			// absolute node_modules paths for every client module on the route.
+			filesBySource.push(["client-module", node.chunks]);
+		}
+		if (filesBySource.length === 0)
+			throw new Error(
+				"Neither entryJSFiles nor clientModules chunk data is available; this manifest layout is not supported.",
+			);
+	} else {
 		throw new Error(
 			"entryJSFiles is unavailable; this manifest layout is not supported.",
 		);
-	const sourceEntries = Object.keys(entries);
-	if (
-		!sourceEntries.some((key) =>
-			key.replaceAll("\\", "/").endsWith(`/app${appPath}`),
-		)
-	)
-		throw new Error("Route page entry is absent from entryJSFiles.");
+	}
 	const bootstrap = filesFrom(
 		buildManifest.rootMainFilesTree?.[appPath] ?? buildManifest.rootMainFiles,
 	);
@@ -110,8 +157,9 @@ export function measureRoute({
 		origins.set(file, sources);
 	};
 	for (const file of bootstrap) add(file, "bootstrap");
-	for (const [entry, values] of Object.entries(entries))
-		for (const file of filesFrom(values)) add(file, entry);
+	const readChunkList = webpackLayout ? assetFilesFromChunkList : filesFrom;
+	for (const [entry, values] of filesBySource)
+		for (const file of readChunkList(values)) add(file, entry);
 	const size = (file, sources) => {
 		const bytes = readAsset(file);
 		return {
@@ -248,12 +296,13 @@ export function collectReport(nextDir, config, metadata = {}) {
 			"Optional PERFORMANCE_COMMIT_SHA supplied by the build caller; not inferred from current checkout.",
 		nodeVersion: process.version,
 		zlibVersion: process.versions.zlib,
-		manifestFormat: "Next App Router client-reference entryJSFiles",
+		manifestFormat:
+			"Turbopack: client-reference entryJSFiles. Webpack: deduplicated clientModules[*].chunks.",
 		definition:
-			"Initial entry envelope: deduplicated rootMainFiles bootstrap plus all entryJSFiles in this route's client-reference manifest, including boundary/loading entries. This is emitted file size, not measured browser traffic or a load-time benchmark.",
+			"Initial entry envelope: deduplicated rootMainFiles bootstrap plus every client chunk this route's client-reference manifest lists, including boundary/loading entries. Turbopack exposes these as entryJSFiles; webpack exposes them only through clientModules[*].chunks, so the same envelope is derived from whichever the build emitted. This is emitted file size, not measured browser traffic or a load-time benchmark.",
 		gzip: "Sum of each unique JavaScript file independently compressed with Node gzip level 9. Excludes HTTP headers and shared-cache reuse.",
 		excluded:
-			"CSS, source maps, images, RSC/HTML payloads, external scripts, async-only chunks absent from entryJSFiles; legacy nomodule polyfills are reported separately.",
+			"CSS, source maps, images, RSC/HTML payloads, external scripts, async-only chunks absent from the manifest's chunk lists; legacy nomodule polyfills are reported separately.",
 		routes,
 	};
 }