feat: add deterministic transaction testing fixtures and documentation

[adnaan-id]

Closes #238

Related Issue

• Closes #

Implementation Scope

• Problem addressed:
• Modules changed: (e.g. src/payments/validation.ts, src/errors/index.ts)
• Public API impact: (none / new export / breaking change — explain if breaking)

Tests Added / Changed

• New tests added: (list files, e.g. tests/validation.test.ts)
• Failure paths covered: (negative inputs, error classification, edge cases)
• Docs-only / config-only PR — tests not applicable because:

Commands Run (local verification)

• npm run verify passed locally

<paste `npm run verify` output summary here>

CI Status

• All CI checks are passing (or red-only for a documented, pre-existing reason)
• CI run link (if needed):

Acceptance Criteria Coverage

• Criterion 1:
• Criterion 2:
• Criterion 3:

Reviewer Notes