Replace cap(parent) - cap(subslice) with a straightforward scan
that compares element addresses: &data[i] == &subslice[0]. This is
well-defined Go pointer comparison on elements of the same backing
array, with no dependency on capacity semantics, reflect, or unsafe.
The scan is O(n) but only runs on error paths, and TOML documents
are small per the project's design constraints.
Also remove the Offset field from ParserError and the setErrOffset
machinery — the offset is computed at the point of consumption
(wrapDecodeError, Parser.Range) rather than cached on the error.
Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>
Cursor Agent
19174a4293