v0.76.0: Contract v2 + content islands (pre-1.0)

The Contract v2 rollup plus content islands, published as one release. cairn is 0.x and pre-1.0; the contract may change again before a stable 1.0.

This consolidates the unpublished 0.69–0.75 development minors plus islands. The last published was 0.68.0, so consumers cross the whole v2 contract in one jump. See the 0.76.0 CHANGELOG entry for every "Consumers must" migration step.

Highlights:

• One fieldset field system for concepts and directive components
• A six-group adapter with concept-owned URL policy (defineConcept)
• Reference fields and structured (object/array) fields
• The Backend interface behind a githubApp(...) provider
• The entry-aware render seam
• Opt-in content islands on a new ./islands subpath

The two production sites stay caret-pinned at 0.68.x until they cut over.