Fix subslice offsets when synthesizing the license field

[devtechedge]

Fixes #314.

The insert position was computed relative to the subslice after the package marker but used to index the full manifest, so it landed five bytes early.

That either panics on a split UTF-8 character or silently corrupts an unrelated value, depending on the manifest.

The fix adds the subslice base back before indexing, and the returned offset still points at the start of the expression.

Two regression tests cover the panic case (multibyte description with CRLF endings) and the silent corruption case, plus a TOML round-trip check.

Note: clippy on a current toolchain reports a pre-existing collapsible_if at src/lib.rs:399, untouched by this change.

[Jake-Shadle]

Thanks!

[devtechedge]

Thanks for merging this, @Jake-Shadle.

Rebasing that subslice-relative insert position was a good reminder of how carefully cargo-about tracks byte offsets so diagnostics keep pointing at the right span.

Glad the multibyte CRLF regression test earned its place.

Thanks for maintaining such a useful tool for the Rust packaging ecosystem.