Replace interfaces with typebox equivalents for return types.
Resolve 6 open design questions:
1. Rebuild over incremental (graph sizes too small to matter)
2. Strict internal-only subgraph (matches graphology-operators)
3. Throw CircularDependencyError on cycles (both consumers treat as bugs)
4. Always propagate through completed nodes, exclude from output only
5. Defer depth-escalation to v2 (multiplicative compounding already captures it)
6. Adopt source->target edge keys from the start
Add section decomposing TaskGraph into thin data class + standalone
analysis functions, with operations pattern at the consumer layer.
