[robhogan]

Reduces storage and upload volume for E2E screenshot testing.

Currently, for simplicity, every push to main uploads a full copy of every screenshot under its own commit - ~3.5MB a push for ~50 tests across both platforms - so storage grows with pushes rather than with visual changes.

This stores each baseline image once at blobs/<sha256>.png, and each commit's manifest refers to its images by hash. A push uploads its manifests plus only the images not already in blobs/, so one with no visual change publishes a few KB. Manifests are still uploaded last, so a baseline only exists once its images do.

Older baselines are still read from their per-commit paths, so there's nothing to migrate. Nothing expires baselines/ or blobs/.

We plan to move storage backends, so I'd land this before we switch and we'll just have a clean base in the new backend. (Note this wouldn't have been possible with backends that enforce expiry, but GCP is fine)

Changelog: [Internal]

Test plan:
New unit tests. Checked against the real bucket that an existing baseline still resolves from its old path, and that a missing blob is detected. The first real publish will be the first push to main after this lands.