Use cap(parent) - cap(subslice) to compute byte offsets between slices
that share a backing array. This is safe pure Go: subslicing preserves
the backing array and adjusts the capacity accordingly, so the
difference in capacities equals the byte offset.
This removes the reflect import from both errors.go and
unstable/parser.go, eliminating the last reflect-based pointer
arithmetic used for error position tracking.
Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>
Cursor Agent
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