fix: measure subpath exports in SIZES.md

Update measure-sizes.js to measure exports from subpath entry
points (styles.js, hydration.js, arrays.js) instead of only
the main barrel. Fixes css showing 0 B after moving to subpath.

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

Author: janechu

packages/fast-element/SIZES.md

@@ -11,10 +11,13 @@ Bundle sizes for `@microsoft/fast-element` exports.
 | observable | 6.74 KB | 2.50 KB | 2.24 KB |
 | attr | 477 B | 288 B | 244 B |
 | children | 4.81 KB | 1.86 KB | 1.64 KB |
-| css | 0 B | 20 B | 1 B |
 | ref | 3.78 KB | 1.53 KB | 1.33 KB |
 | slotted | 4.60 KB | 1.79 KB | 1.58 KB |
 | volatile | 6.79 KB | 2.53 KB | 2.25 KB |
 | when | 1.82 KB | 712 B | 568 B |
 | html | 25.92 KB | 8.50 KB | 7.61 KB |
 | repeat | 29.57 KB | 9.42 KB | 8.47 KB |
+| css | 1.55 KB | 731 B | 654 B |
+| ElementStyles | 788 B | 397 B | 332 B |
+| enableHydration | 24.27 KB | 7.59 KB | 6.86 KB |
+| ArrayObserver | 12.51 KB | 4.45 KB | 4.01 KB |

packages/fast-element/scripts/measure-sizes.js

@@ -15,7 +15,6 @@ const namedExports = [
     "observable",
     "attr",
     "children",
-    "css",
     "ref",
     "slotted",
     "volatile",
@@ -24,6 +23,13 @@ const namedExports = [
     "repeat",
 ];
 
+const subpathExports = [
+    { name: "css", path: "@microsoft/fast-element/styles.js", export: "css" },
+    { name: "ElementStyles", path: "@microsoft/fast-element/styles.js", export: "ElementStyles" },
+    { name: "enableHydration", path: "@microsoft/fast-element/hydration.js", export: "enableHydration" },
+    { name: "ArrayObserver", path: "@microsoft/fast-element/arrays.js", export: "ArrayObserver" },
+];
+
 function formatBytes(bytes) {
     if (bytes < 1024) return `${bytes} B`;
     return `${(bytes / 1024).toFixed(2)} KB`;
@@ -39,8 +45,8 @@ function measureBuffer(buffer) {
     };
 }
 
-async function measureExport(exportName) {
-    const contents = `import { ${exportName} } from "@microsoft/fast-element";\nexport { ${exportName} };\n`;
+async function measureExport(exportName, importPath = "@microsoft/fast-element") {
+    const contents = `import { ${exportName} } from "${importPath}";\nexport { ${exportName} };\n`;
 
     const result = await build({
         stdin: {
@@ -81,6 +87,15 @@ async function main() {
     );
     results.push(...exportResults);
 
+    // Measure subpath exports in parallel
+    const subpathResults = await Promise.all(
+        subpathExports.map(async ({ name, path: importPath, export: exportName }) => {
+            const sizes = await measureExport(exportName, importPath);
+            return { name, ...sizes };
+        }),
+    );
+    results.push(...subpathResults);
+
     // Generate markdown table
     const lines = [
         "# Export Sizes",