Add transaction template system for common patterns

[Just-Bamford]

Problem

Building similar transactions repeatedly is tedious. No way to save and reuse transaction templates.

Solution

Implement transaction templates: \saveTransactionTemplate(name, template), \loadTemplate(name, params?)\ in \src/transaction/index.ts.

Acceptance Criteria

• Template save/load functions added
• Store templates in memory or persistent store
• Apply parameters to templates
• Tests cover template workflows

Note for contributors

Design template structure. Support parameter substitution. Add tests to \src/tests/transaction.test.ts.