Versioned client <-> engine decision contract (+ contract tests)

[ramboz]

Part of #66

Problem

The client and the engine/worker exchange decisions (audience resolutions, experiment assignments, fragment URLs) with an implicit shape. Nothing pins it down, so any conforming worker/engine has to be reverse-engineered against the client.

Proposal

Define a small, versioned normalized decision contract — the shapes exchanged for:

• audience resolution ({ [name]: boolean }, or richer),
• experiment assignment ({ [experimentId]: variant }),
• rendered decision (URL / JSON / ref),
• the request context ({ visitorId, url, consent, geo, … }).

Ship contract tests validating those shapes on both ends via shared fixtures, so any conforming worker/engine works out of the box.

Acceptance criteria

• Documented, versioned contract for the decision + context shapes.
• Shared fixtures + contract tests exercised by the client resolver.
• Referenced by the reference worker.