Three spec-compliance fixes to PathParser:
1. Q-construct regex now accepts lowercase letters (qAo, qAr, etc.).
The moduledoc lists qAo as a Q-construct to skip but the previous
~r/^qA[A-Z]$/ rejected it, leaving it to be misclassified.
2. self_loop?/2 now compares callsigns only; the spurious
src_pos == dst_pos clause would falsely suppress two distinct
stations that happen to share coarse coordinates (e.g. same building).
3. Strengthen the "lookup returned nil → drop hop, do NOT advance source"
test: extend the path to three tokens so the assertion actually proves
the third hop chains from the previous good source rather than from
the dropped one. The two-token version passed regardless of behavior.
Self-loop fixture updated to trigger via callsign equality (the only
remaining mechanism) instead of position equality.
Graham McIntire
4759d1313e