Structure Smart CDN image candidates

[kvz]

Why

The responsive-image helper currently returns preformatted srcset strings, coupling the signing
primitive to one renderer. Its only application consumer is still under our control, so this is the
right moment to replace that two-day-old contract with a renderer-neutral model before broader
adoption.

What changed

• return ordered structured sources containing format, quality, URL, and width data
• export the candidate, source, format, and result types
• retain deterministic signing, format ordering, width deduplication, and validation behavior
• document the new result and ship it as an intentional patch-level replacement

Verification

• corepack yarn workspace @transloadit/utils check
• corepack yarn check
• corepack yarn verify:full
• corepack yarn release:pack:dry-run
• council review; the sole semver finding was explicitly accepted for this newly introduced,
  zero-external-consumer API