Call nodes under //line directives get empty byte spans #21

Describe the bug
Call nodes in a file with a //line directive get an empty byte span. Their positions come from the directive-adjusted line:col, while byte offsets index the real file, so slicing module.source returns an empty string.

To Reproduce

1. A Go file containing //line virtual.y:900:1 followed by a function that makes calls.
2. cango -i <dir> -a 1 -o /tmp/out
3. Inspect those call nodes' span.bytes: start equals end (e.g. [433,433]).

Expected behavior
span.bytes covers the call's real text in the file, so source slicing recovers it. The line:col can stay directive-adjusted or be real, as long as the doc says which.

Logs
Observed in the testdata/dataflow/linedir fixture added by PR #16.

Additional context
Affects generated code: goyacc, ragel, templ, cgo output. Predates PR #16. The call callee resolution under //line is fixed in that PR; only the span is still wrong.