docs: prefer fallible construction in public examples

[blackopsrepl]

Summary

• update public docs and examples to prefer try_new for user-provided coordinates and bounds
• keep panic-free examples aligned across the crate docs and README
• preserve guidance that infallible constructors are still fine for trusted literals

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --doc
• cargo test

Fixes #12

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Rebind coord to a Coord before the later example calls

Replacing the old let coord = Coord::try_new(...).unwrap(); with let trusted_coord = Coord::new(...) leaves coord bound to the Result<Coord, CoordError> from a few lines above. The subsequent coord.distance_to(other) and coord.into() therefore no longer type-check, so the public Coord example in the README regressed from copy-pastable to uncompilable.

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