Invoice Split Preview Change Diff Generator

[Kingsman-99]

Description

When an invoice's split configuration is updated (e.g. a recipient is added, shares are rebalanced), the SDK replaces the entire SplitConfig without showing what changed. Invoice issuers and auditors need to see a typed diff between two split configurations — which recipients were added, removed, or had their ratio changed — before confirming the update. A change diff generator should produce this comparison on demand.

Technical Context

Extend src/splitPreview.ts. Accepts two SplitConfig objects (original and revised). Produces SplitConfigDiff added to src/types.ts with fields: added: RecipientShare[], removed: RecipientShare[], changed: { accountId, oldRatio, newRatio }[], totalRatioDelta. Integrates with src/diff.ts for the generic structural diff utilities. Exposes a generateSplitDiff(original, revised): SplitConfigDiff function exported from src/index.ts.

Acceptance Criteria

• generateSplitDiff(original, revised) correctly identifies recipients added in revised but absent in original
• Correctly identifies recipients present in original but absent in revised as removed
• Correctly identifies recipients present in both with different ratios as changed, with oldRatio and newRatio in the result
• totalRatioDelta is the signed difference in sum of ratios between the two configs (should be 0 for valid rebalances)
• Unit tests cover: no change, single add, single remove, ratio rebalance, simultaneous add and remove
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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