Fix child-workflow conformance worker registration

[rmcdaniel]

Summary

• give the child-workflow probe synthetic workers the structured portable-affinity refusal manifest required by protocol 1.19
• add a focused source-contract regression assertion

This changes only conformance-driver code. The frozen 2.0.0-rc.69 product image remains the artifact under test and does not need to be republished.

Verification

• PHP syntax check in the published Server runtime
• focused PHPUnit test: 1 test, 79 assertions

Closes #65.