Make config graph children first-class

[pinodeca]

Summary

• move IF/LOOP conditions and additional JOIN branches into first-class opaque Durofut child fields
• keep df.nodes.query unchanged by materializing config children back to node ID references
• remove repeated escaping and the remaining serde recursion ceiling for config-nested graphs
• document the transient envelope versus durable FunctionNode boundary

Compatibility

This changes the unversioned transient Durofut JSON envelope. It does not change the df.nodes schema, persisted node query format, in-flight instances, or duroxide replay state. No extension upgrade script is required.

Validation

• cargo fmt -p pg_durable -- --check
• cargo clippy --features pg17 --lib -- -D warnings
• ./scripts/test-unit.sh (265 passed, 16 ignored)
• ./scripts/test-e2e-local.sh 02_conditionals
• ./scripts/test-e2e-local.sh 09_graph_and_validation
• focused regressions for linear nested-condition growth and IF/JOIN config ID materialization