test: add unit tests for the asset map hooks

[stevenle]

Adds test coverage for the useAssetUrl() hook introduced in #1334, which this
branch targets so the diff is tests only.

src/core/hooks/useAssetMap.test.tsx

Renders components through a stub AssetMap and asserts:

• useAssetUrl() resolves a src to its compiled asset URL, run against both
  renderers (preact-render-to-string and @blinkk/root/jsx), since the
  renderer is picked by the jsxRenderer config in root.config.ts.
• A leading / on the src is normalized away.
• Unknown srcs and assets with an empty assetUrl throw.
• useAsset() returns the asset from the module graph.
• useAssetMap() throws when rendered outside of ASSET_MAP_CONTEXT.

src/render/asset-map/dev-asset-map.test.ts

Covers the path containment checks in DevServerAssetMap.get(), which changed
from file.startsWith(rootDir) + the async directoryContains() in fsutils
to a local synchronous helper:

• Files inside the project resolve to a root-relative URL.
• A sibling dir that shares the root dir's path prefix (<ws>/site-legacy vs.
  <ws>/site) is served from /@fs/ instead of producing the truncated
  -legacy/main.ts URL the old prefix check returned.
• Workspace files that don't exist on disk resolve instead of throwing
  ENOENT, which the old realpath()-based check did.
• Files outside the workspace return null and log.

The suite builds a temp workspace with a pnpm-workspace.yaml so vite's
searchForWorkspaceRoot() resolves deterministically.

Testing

pnpm test in packages/root: 38 files / 216 tests passing. pnpm lint is
clean. No changeset — tests only, and #1334 already carries one.

Generated with Claude Code.

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