Add route scoring to select the optimal path in WaterfallRouter

[Kingsman-99]

Description

WaterfallRouter tries routes in declaration order and returns the first successful one. It does not score routes by latency, fee, or reliability, so the first-declared route is always tried even when a more efficient one is available.

Acceptance Criteria

• Each route definition gains an optional score?: number field (higher = tried first; default 0)
• Routes are sorted by score descending before waterfall execution
• Ties in score fall back to declaration order
• Unit tests pass

Context

• Target file: src/routing/WaterfallRouter.ts
• Sorting must be stable to preserve FIFO among equal-score routes