fix(split): reject invalid runtime ratio legs

[woahwhattheheck]

Closes #1012

Summary

• validate every declared split ratio as a runtime number, finite, and within [0, 1]
• distinguish an omitted ratio (undefined) from explicit null and other wrong-type values
• fail closed on malformed/non-finite values without changing the existing SplitRatioSumError code/fields
• preserve floating-point tolerance, omitted-ratio contribution, and amount-only split behavior
• add focused regressions proving hostile values fail before the capacity preflight

Scope

Two files only, based on exact upstream main@230fe3f67e806fbd0f0e67a29bd7b90ffec80789:

• src/payments/splitExecutor.ts
• test/splitExecutor.invalid-ratios.test.ts

The earlier competing PR #1015 is closed unmerged. This carrier implements the repair already documented and claimed in #1012, including the additional runtime null/wrong-type escape found during source review.

Validation

Immutable branch readback confirms the runtime type guard, finite/range checks, NaN-safe aggregate comparison, explicit null/wrong-type regressions, preflight blocking, and a 2-ahead / 0-behind exact-base branch.

Prior focused validation of the narrower issue-body candidate was 10/10; this environment has not executed the new runtime-type regressions, so no fresh full-suite/CI-green claim is made here. Hosted CI should be treated as authoritative for the published head.

Claim / compensation

The assignment and payment request is already live on issue #1012 under woahwhattheheck. This PR is the implementation carrier for that claimed contribution. Please evaluate it under the issue's applicable reward/bounty process. No award or payment receipt is asserted.