ci(sdk): wire binary-compatibility-validator — committed .api dumps + blocking apiCheck gate

[ThomasGorisse]

Shortlist #2642 item 4 (V1, roi 8.5 / quality 9.5, no gate): the SDK has no public-API surface tracking — a signature change in sceneview, arsceneview, or sceneview-core ships silently unless a reviewer happens to notice. For an AI-first SDK whose docs (llms.txt) promise exact signatures, that is the doc-drift bug class at the binary level.

Scope

• Wire Kotlin's binary-compatibility-validator into the root build for the published Android/KMP modules (sceneview, arsceneview, sceneview-core; evaluate sceneview-web JS surface separately if unsupported).
• Generate and COMMIT the initial .api dumps (apiDump).
• Add a blocking apiCheck step to ci.yml (same tier as unit tests — a red apiCheck fails the PR).
• Document the workflow in CONTRIBUTING.md: intentional API changes re-run apiDump and commit the diff, which makes the public-API delta reviewable in the PR.
• Check check-doc-drift.sh interplay: the .api diff is a deterministic signal it can consume later (note only, no coupling required now).

Verified absent on main as of 2026-07-16 (grep binaryCompatibilityValidator|apiValidation|apiCheck across build files: no match).