This is crucial documentation that outlines the PSBT format, which will
help guide implementation of the CLI parser for Bitcoin raw transactions.

Issue: BTC-2652

Co-authored-by: llm-git <llm-git@ttll.de>

Simplify the test file structure by removing the redundant tests module,
as the file is already within a tests module.

Issue: BTC-2652

Co-authored-by: llm-git <llm-git@ttll.de>

Implement a new parser module for examining low-level PSBT structure with
raw key-value pairs as defined in BIP-174. This exposes internal details
like proprietary keys, raw type IDs, and byte structure that standard
parsers might hide.

Also add test utilities for loading PSBT fixtures across different networks
and signature states.

Issue: BTC-2652

Co-authored-by: llm-git <llm-git@ttll.de>

Reorganize code structure with better modularization and add transaction
parsing capability. This includes:
- Create a shared input handling module
- Support for transaction parsing via new `tx parse` command
- Move parsers into organized submodules
- Improve file reading with better error handling
- Add support for decoding different input formats (hex/base64/raw)
- Fix clippy warnings in existing code

Issue: BTC-2652

Co-authored-by: llm-git <llm-git@ttll.de>

Simplified the fixture handling code and tree rendering utilities. Moved
non-test-related code behind #[cfg(test)] attributes to reduce binary size.
Removed unused color schemes and unnecessary public exports.

Issue: BTC-2652

Co-authored-by: llm-git <llm-git@ttll.de>