Trace: a snap's "on a permitted value" test compares raw pairs, while table rows compare values

[christianparpart]

What is inconsistent

To decide whether a value sits on a row or between two, a trace compares two bounds written as numerator/denominator pairs. With #12, a lookup's segment and a missed lookup's covered range compare them by value, through detail::same_declared_bound in include/formula-cpp/trace_render.hpp, so 2/4 and 1/2 are one row. A snap's "on a permitted value" test does not. snap_suffix (around trace_render.hpp:2164) compares the raw pairs:

if (neighbours.low == neighbours.high)

So two neighbours equal in value but stored as different pairs read as two neighbours, not "on a permitted value".

Expected

Do one of these:

• If a snap's permitted values can hold the same value as two different pairs, compare by value, as table rows do, using same_declared_bound.
• If they cannot (if the permitted set is validated as strictly ascending by value where it is declared), say so in a comment at that comparison, so the difference from the table rows reads as intended.

Also

lookup_miss_text spells lowText before it branches, and the two-row path never uses it. Spell it only where it is used.