Add internal template parser for custom error derive

[RAprogramm]

Summary

• add a public error::template module that parses #[error("…")] strings into reusable segments and metadata for future derive work
• implement the parser and formatting helpers, plus comprehensive unit tests, under src/error/template
• document the new building block in the README/CHANGELOG and expose the module from the crate root

Testing

• cargo +nightly fmt --
• cargo clippy -- -D warnings
• cargo build --all-targets
• cargo test --all
• cargo doc --no-deps

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https://chatgpt.com/codex/tasks/task_e_68cbe29f29b0832b8d2297f489959605