pathRouter.ts: Add fallback route when primary payment path is exhausted

[Kingsman-99]

Description

src/pathRouter.ts finds payment paths but returns an empty result when no path meets the minimum liquidity threshold. Adding a fallback that relaxes the threshold by a configurable percentage gives callers a viable (if more costly) route instead of a hard failure.

Acceptance Criteria

• findPath(params) accepts a fallbackSlippagePct?: number option (default: none)
• When no path meets the primary threshold, the router retries with threshold * (1 + fallbackSlippagePct/100)
• The returned path, when from the fallback, includes a usedFallback: true flag
• Unit tests pass

Context

• Target file: src/pathRouter.ts
• The fallback retry is a single retry; do not implement multi-level fallback chaining