Add header license REUSE check

[sfraczek]

Automatic tool should check if license header exists in source files.

Related issue #133

[Copilot]

Pull Request Overview

This PR implements REUSE compliance by adding standardized license headers to all source files and configuration files. This addresses issue #133 to ensure proper license attribution throughout the codebase.

• Adds SPDX license headers to all Rust source files and configuration files
• Creates REUSE.toml configuration file for license management
• Adds GitHub Actions workflow for automated REUSE compliance checking
• Includes standard license text files and removes old LICENCE file

Reviewed Changes

Copilot reviewed 23 out of 27 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/*.rs	Added SPDX license headers to all Rust source files
REUSE.toml	Configuration file defining license annotations for different file types
.github/workflows/reuse-check.yml	New workflow for automated REUSE compliance checking
LICENSES/*.txt	Standard license text files for BSD-3-Clause and CC0-1.0
README.md, Cargo.toml, .github/*.yml	Added license headers to configuration and documentation files
LICENCE	Removed old license file in favor of standardized LICENSES directory

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[Copilot]

The license header should be placed at the very beginning of the file, before any other directives including #![cfg(...)]. This ensures the license information is the first thing seen in the file.

Suggested change

	#![cfg(feature = "gui")]
	// SPDX-FileCopyrightText: 2025 RustInFinance
	//
	// SPDX-License-Identifier: BSD-3-Clause
	// SPDX-FileCopyrightText: 2025 RustInFinance
	//
	// SPDX-License-Identifier: BSD-3-Clause
	#![cfg(feature = "gui")]

[sfraczek]

@jczaja

• how do you want to deal with YEARS?
• What about the copyright holder: RustInFinance or leave Jacek Czaja as it was?

[tsocha]

Not a fan of these empty lines, can you remove it?

[sfraczek]

It should be doable with templates:
Source fsfe/reuse-tool#562