Prepare display spec scaffolding for format args

[RAprogramm]

Summary

• extend the derive input model with FormatArgsSpec, FormatArg and FormatBindingKind, plus new display variants for template arguments and fmt = <path>
• update the display generator and error trait expansion to recognise the new display variants while deferring custom formatter support with diagnostics
• bump crate versions to 0.5.14/0.1.6 and refresh the changelog and README snippets for the new release

Testing

• cargo +nightly fmt --
• cargo +1.90.0 clippy -- -D warnings
• cargo +1.90.0 build --all-targets
• cargo +1.90.0 test --all
• cargo +1.90.0 doc --no-deps
• cargo deny check
• cargo audit

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https://chatgpt.com/codex/tasks/task_e_68cd3dee0d50832babd7325204790958