feat(sdk): mark fallible calls as must use

[lntutor]

Summary

• add an explicit #[must_use] diagnostic to all 38 public fallible SDK calls
• use one consistent message so ignored results explain how callers should respond
• leave infallible SDK functions and runtime behavior unchanged

Fixes #85

Verification

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

[GianIac]

Thanks! If you'd like to contribute to other things as well, you're more than welcome!