feat(bucketing)!: anchored bucketing layout (contract v12)

Introduces the anchored bucketing layout as a version-gated pass
(experience.version > 11) beside the existing packed walk, and extends the
public BucketingManagerInterface with getBucketRanges, selectBucketAnchored,
and getBucketForVisitorAnchored. Cross-SDK parity with the JavaScript SDK
reference is enforced by the shared golden-vector fixture.

Ships inert: the backend currently stamps every experience version 11, so no
served experience routes to anchored until the coordinated backend bump to 12.
The packed path (version <= 11) is bit-for-bit unchanged.

BREAKING CHANGE: bucketing contract v12 introduces the anchored layout and
extends the public BucketingManagerInterface. Consumers implementing that
interface must add the new methods. Runtime behavior is unchanged until the
backend stamps experience.version > 11.

Author: abbaseya