Documentation: add ADR-005 for fee-bump transaction design

[Ejirowebfi]

Description

Fee-bump transactions are documented in README.md but there is no Architecture Decision Record explaining why this approach was chosen over alternatives (e.g., sponsored reserves, pre-funded accounts).

Tasks

• Create docs/adr/ADR-005-fee-bump-transactions.md
• Document the problem, considered alternatives, and decision rationale
• Link the new ADR from README.md
• Follow the format of existing ADRs in docs/adr/

Acceptance Criteria

• ADR file exists at the correct path
• It covers: context, decision, alternatives considered, consequences
• README.md links to the new ADR