Document SyncBackend applyOps visibility contract

[marcus-pousette]

Summary

• Documents SyncBackend.applyOps as a read-after-write boundary.
• Adds an explicit backend contract assertion that applyOps results are visible through listOpRefs, getOpsByOpRefs, and maxLamport after resolve.
• Removes a stale conformance polling path that implied async backends could resolve sync before applyOps was visible.
• Clarifies that FlushableSyncBackend is only a test/benchmark utility, not part of the normative sync backend contract.

Fixes #112.