Use indexed watch folder paths for external assets

[robhogan]

Summary

Assets outside projectRoot currently derive their httpServerLocation from a path relative to projectRoot. This can produce .. paths which escape /assets, and does not identify which configured watch folder owns the asset.

This uses Metro's existing [metro-watchFolders]/N/ namespace for external asset URLs:

• asset transforms and getAssets encode the path relative to the containing watch folder
• the encoded path is included in the transform cache key
• the /assets endpoint resolves the watch folder prefix before selecting platform and density variants
• assets within projectRoot retain their existing URLs

Keeping these requests under /assets preserves Metro's platform-specific and density-aware asset resolution.

Fixes #19
Fixes #290

Changelog: [Fix] Use indexed watch folder paths for assets outside projectRoot

Test plan

Created a minimal project which imports an asset from a sibling watch folder:

metro-watch-folder-e2e/
├── project/
│   ├── AssetRegistry.js
│   └── index.js
└── shared/
    └── media/
        ├── tone.mp3
        ├── tone@2x.mp3
        ├── tone@1x.ios.mp3
        └── tone@2x.ios.mp3

index.js imports the sibling asset:

const asset = require('../shared/media/tone.mp3');

Started Metro from this checkout with shared configured as a watch folder:

$ yarn start serve \
    --config /Users/robhogan/cowork/metro-watch-folder-e2e/metro.config.js \
    --host 127.0.0.1 \
    --port 8099 \
    --reset-cache

Metro ready

Requested an iOS bundle and inspected the generated asset metadata:

$ curl --fail --silent --show-error \
    --output /private/tmp/metro-watch-folder-bundle.js \
    'http://127.0.0.1:8099/index.bundle?platform=ios&dev=true&minify=false'

$ rg -A 5 '"httpServerLocation"' /private/tmp/metro-watch-folder-bundle.js
    "httpServerLocation": "/assets/[metro-watchFolders]/1/media",
    "scales": [1, 2],
    "hash": "6db41d4f75e3e7e47d792c57abc17cb9",
    "name": "tone",
    "type": "mp3"
  });

Metro normalises projectRoot as watch folder 0, so the configured sibling directory is watch folder 1.

Requested the emitted asset path at 2x density for iOS:

$ curl --silent --show-error --globoff --include \
    'http://127.0.0.1:8099/assets/[metro-watchFolders]/1/media/tone@2x.mp3?platform=ios&hash=6db41d4f75e3e7e47d792c57abc17cb9'

HTTP/1.1 200 OK
Content-Type: audio/mpeg
Content-Length: 7

ios-2x

The response body matches tone@2x.ios.mp3, confirming that the indexed URL resolves through the sibling watch folder while retaining platform and density selection.

Also ran:

$ yarn jest packages/metro/src packages/metro-transform-worker/src --runInBand
Test Suites: 63 passed, 63 total
Tests:       1 skipped, 900 passed, 901 total
Snapshots:   160 passed, 160 total

$ yarn flow check --temp-dir /private/tmp/metro-flow-pr19
$ yarn typecheck-ts
$ yarn verify-api-snapshots
$ yarn lint

All checks passed.