[#454] Add bounded atomic route batches

[Jagadeeshftw]

Summary

• add bounded compute_route_fees batch execution with explicit empty/oversize guards
• preflight every route before accounting and reuse the canonical single-route policy
• preserve all-or-nothing liquidity, counters, timestamps, and event behavior
• add focused regression tests and batch-operation security guidance

Acceptance criteria

• Valid batches apply all entries and emit the existing per-item route events.
• Any invalid item rejects the complete transaction.
• Oversized batches return the typed BatchTooLarge error.
• Empty batches return the typed EmptyBatch error.
• Per-item validation matches the single-route path.

Validation

• cargo test --lib batch_route_tests (5 passed)
• rustfmt --edition 2021
• git diff --check
• 503 changed lines

Closes #454