Refactor SpanPair to preserve zero-length location semantics

[Copilot]

Please run scripts/bootstrap_test.sh and push all of the changes it make before making the PR.

This will run brief tests, and make it easier to look at what part of generated code has been changed.

SpanPair previously used Option<(usize, usize)>, which collapsed zero-length and missing-context cases. This change makes zero-length spans explicit as (pos, pos) so identical length can still carry distinct source positions.

• Span storage model

  • Changed SpanPair.pair from Option<(usize, usize)> to (usize, usize).
  • Updated constructors/converters to operate on concrete ranges.

• Zero-length position semantics

  • For len == 0, location now resolves to the end position of the most recent token as (e, e).
  • This preserves positional meaning for empty spans instead of erasing it.

• Fallback behavior

  • When there is no stack context, fallback remains (0, 0).
  • Byte-range → Span conversion remains centralized via byte_range_to_span.

#[derive(Clone, Debug, Copy)]
pub struct SpanPair {
    /// zero-length spans are represented with equal values `(pos, pos)`.
    pub pair: (usize, usize),
}

if len == 0 {
    if let Some(after_pos) = stack.next() {
        let (_, e) = after_pos.pair;
        return SpanPair { pair: (e, e) };
    }
    return SpanPair::default();
}