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

[woahwhattheheck]

Submitted a PR for this: #836

WaterfallRouter turned out to sequence payout tiers rather than race
alternative routes, so I read "routes" as config.tiers and "declaration
order" as tier order — the scoring and stable-tie requirements map onto it
directly. Say the word if you meant a different structure.

Two points behind the implementation:

The ordering is total, not just stable. Array.prototype.sort has been
stable since ES2019, but the comparator also breaks ties on the original
index, so equal-scored tiers keep FIFO order regardless of engine or compile
target instead of resting on that guarantee. Sorting happens on a copy, so a
caller's tier array is never reordered.

A non-finite score throws. NaN makes a comparator return NaN, which is
neither negative, zero, nor positive — the sort then yields an arbitrary order
with no error, silently making the funding sequence undefined. It is validated
next to the existing minimumAmount check and raises ValidationError.

A config with no scores behaves exactly as before; there is a test pinning
that.

18/18 tests pass (9 pre-existing plus 9 new). tsc --noEmit shows the same
210 errors as unmodified main, none in either changed file.

[woahwhattheheck]

I'd like to work on this — requesting assignment.

I should be straight about the order of events: I opened PR #836 for this
issue before asking. CONTRIBUTING is explicit that coding should not start
until a maintainer assigns the issue, and I did not read it before starting.
That is my mistake, not a grey area.

The PR stands on its own if it is useful to you, but I am not claiming any
standing from it. If you would rather assign this to someone else, say so and
I will close it — no need to explain.

If you do assign me, I will re-push under your branch convention as
feat/issue-777-waterfall-route-scoring; my current branch omits the issue- prefix.