feat(migration): support monorepo peerDependencies detection

Find the nearest package.json for each source file instead of only
checking the root. This allows monorepos to have different packages
with different peerDependencies - a vite plugin package can have vite
in peerDependencies (imports preserved) while an app package without
peerDependencies will have imports rewritten.

- Add find_nearest_package_json() to walk up directories
- Cache package.json lookups for performance
- Add monorepo unit tests and snap-test

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: fengmk2

.github/workflows/e2e-test.yml

@@ -9,6 +9,10 @@ on:
       - main
     paths-ignore:
       - '**/*.md'
+  pull_request:
+    paths:
+      - 'ecosystem-ci/**'
+      - '.github/workflows/e2e-test.yml'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.sha }}
@@ -88,6 +92,14 @@ jobs:
               vite run lint
               vite run type
               vite run test -- --coverage
+          - name: vite-plugin-react
+            node-version: 22
+            command: |
+              vite run format
+              vite run lint -- --fix
+              # TODO(fengmk2): run all builds and tests after tsdown version upgrade
+              vite run @vitejs/plugin-rsc#build
+              vite run @vitejs/plugin-rsc#test
 
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2