refactor: rename SupplyChain Shipment.source_node/target_node to source/target

[rororowyourboat]

Problem

Every flow-like element across the ecosystem uses source and target:

• StockFlow Flow: source, target
• DFD DataFlow: source, target
• VSM flows: source, target

But SupplyChainModel's Shipment uses source_node and target_node:

# packages/gds-business/gds_business/supplychain/elements.py:29-30
class Shipment(BaseModel, frozen=True):
    source_node: str   # inconsistent
    target_node: str   # inconsistent

Fix

Rename to source / target for consistency. Update validators in supplychain/model.py:65-74.

This is a breaking change for gds-business 0.1.0.