Reserve image layout during signing and complete responsive examples

[kvz]

Why

Direct images have required source dimensions but used to reserve no space while request-time signing suspended. The responsive examples also supplied sizes without the corresponding CSS, and did not demonstrate an appropriately small avatar candidate set.

Changes

• Use an inert, invisible, source-free native image as the default Suspense fallback, retaining the final image's layout attributes and styles without signing or fetching early. Explicit fallbacks (including null) remain overrides.
• Complete the 2400×1600 hero recipe with a maximum 960px display width, automatic height and matching sizes. Add a 48px avatar with 48/96px candidates. Apply matching CSS to the packed fixture.
• Explain source proportions, CSS cropping versus encoded padding, and the opt-in hydration workaround outside the happy path.

Verification

• Reproduced the missing shell with a failing pending-connection unit test and failing packed-fixture assertion before implementation.
• yarn workspace @transloadit/img check: 75 tests plus type fixtures.
• yarn test:img:fixture: packed production Next build, shell geometry attributes/CSS, avatar candidates, signing, authorization, tampering and secret exclusion.
• Separate local Chromium geometry probe using actual suspended/resolved component markup and local SVG stand-ins for decoded bytes: hero 960×640 at viewport 1200 and 375×250 at viewport 375; avatar 48×48 in both. Following-content coordinates matched exactly before/after resolution. The shell contained no image source requests and its placeholders were absent from the accessibility tree. Probe artifacts retained locally; own browser closed.
• PR A also passed full repository yarn check and all GitHub checks.

This is PR B, stacked on #497. No merge/publication. The geometry probe is not the later real-CDN/browser authorization suite; the orchestrator owns council review after the round.

[kvz]

Superseded by #500, which now carries the whole change against main.