Add a fake schedule adapter contract suite

[jonathanmcmichael]

Outcome

Create a deterministic fake ScheduleAdapter that proves capability declarations, identity-preserving import, plan-before-apply behavior, and explicit unsupported operations without using a vendor SDK.

Truth source

The fake imports two activities and one finish-start relationship with fixed Rustit UUIDs plus external IDs A1000 and A1010. Its export plan reports one update, zero creates/deletes, and no warnings.

Invariants

• external IDs remain ExternalIdentity values
• the canonical schedule does not gain P6/OPC/SYNCHRO fields
• capabilities accurately describe behavior
• plan_export does not mutate external state
• unsupported apply returns AdapterError::Unsupported

Expected scope

Add test support and a Sync Lab fixture against the existing adapter traits.

Non-goals

Network calls, credentials, vendor payload schemas, retry policy, and a production adapter.

Acceptance and evidence

• import produces exactly the documented canonical schedule and identity mappings
• plan is deterministic and non-mutating
• declared unsupported apply fails explicitly
• repeated import preserves fixture UUIDs
• cargo xtask verify passes

Dependencies: #10 and the existing adapter contracts.