E2E: add Unity EditMode verification test to CI pipeline

## Context

PR #266 added an e2e test (`test/e2e/conversion.spec.ts`) that exercises the full conversion pipeline with real Unity and mock S3. It also includes a Unity EditMode test (`AssetBundleVerificationTest.cs`, category `E2EVerification`) that is meant to run after the Node e2e test and verify:

- Both Cube bundles from Scene 1 and Scene 2 load successfully
- Both have the **same mesh** (same `Cube.gltf` source — same vertex/triangle count)
- Both have **different textures** (different `albedo.png` — pixel data comparison)

The Unity test exists in the codebase but **is not yet wired into the e2e CI pipeline**. Currently `AssetBundleVerificationTest.cs` downloads bundles from the CDN with hardcoded entity IDs and a fixed `mac` target, which won't work in CI where:

1. The bundles are in mock S3 on disk, not the CDN
2. The build target varies per matrix entry (webgl/windows/mac)
3. Entity IDs are resolved dynamically at test time

## What needs to happen

- Wire the Unity EditMode test into the e2e CI step (after the Node jest run, before the existing `ci-editmode-test.sh`)
- Pass the bundle paths from the Node e2e output to the Unity test (e.g., via a JSON file written by the jest test)
- Make the Unity test load bundles from mock S3 disk paths instead of downloading from CDN
- Handle the build target mismatch (webgl bundles can't load in Editor — may need to skip or use a compatible target)

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