diff.ts: Write tests for three-way merge conflict detection

[Kingsman-99]

Description

src/diff.ts implements three-way diff/merge for invoice state reconciliation, but only two-way diff tests exist. The three-way merge conflict path — where both the local and remote branches have modified the same field — has no coverage.

Acceptance Criteria

• Test: both branches modify the same field → MergeConflict error is thrown with the conflicting field name
• Test: one branch modifies a field, the other does not → merge succeeds with the modified value
• Test: neither branch modifies a field → merge succeeds with the base value
• Unit tests pass

Context

• Target file: src/diff.ts
• Tests should live in src/__tests__/diff.test.ts