Detect negative-weight edges in the payment graph checker

[Kingsman-99]

Description

PaymentGraphChecker validates the structure of payment graphs but does not detect edges with negative weights (amounts). A negative-weight edge can cause unbounded fund extraction when the graph is traversed greedily.

Acceptance Criteria

• checkGraph(graph) returns a validation failure when any edge has weight < 0
• The failure message names the offending edge (source to target)
• Valid graphs with all non-negative weights continue to pass
• Unit tests pass

Context

• Target file: src/graph/PaymentGraphChecker.ts
• Zero-weight edges are allowed (they represent pass-through hops)