Scope coverage to our own sources

v8 coverage had no include, so it walked the entire project root. With the
read-only reference_code checkout present that pulled 572 of 1031 entries into
the report and reported 23.94% for "All files" - a number describing the
reference, not this project. Restricting it to packages/*/src puts the figure
at 59.71%.

Nothing failed on this before because there are no coverage thresholds, but the
report was misleading anyone reading it.

vitest.config.ts

@@ -13,5 +13,13 @@ export default defineConfig({
   test: {
     include: ['tests/**/*.test.ts'],
     setupFiles: ['tests/setup.ts'],
+    coverage: {
+      // Without an explicit include, v8 coverage walks the whole project root.
+      // While the read-only reference_code/ checkout was present that pulled in
+      // ~570 of its files and diluted every percentage below.
+      provider: 'v8',
+      include: ['packages/*/src/**'],
+      exclude: ['**/vendor/**', '**/*.d.ts'],
+    },
   },
 })