Add typed control-flow macros for early error returns

[RAprogramm]

Summary

• add ensure! and fail! macros that provide allocation-free, typed early returns with dedicated tests
• surface the new control-flow helpers throughout the crate documentation and both READMEs
• bump the crate version to 0.21.0 and record the release in the changelog

Testing

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

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https://chatgpt.com/codex/tasks/task_e_68d3aa75765c832b913e0353f21824c3