sl0p: warn when the responsive layout has no swap layout to collapse into

The collapse selects a swap layout BY NAME, so a tab with no swap layouts
simply does nothing when the terminal gets narrow -- no error, no hint. The
usual cause is not the responsive settings at all: a custom layout file only
gets swap layouts from a sibling "<layout>.swap.kdl", so launching with one
that has no such file silently disables the feature.

That is the second time this feature has failed invisibly (the first was a
layout marked damaged), and both cost real debugging time. Logs once per tab,
not per resize, and names the actual cause rather than the symptom.

0 files changed, 0 insertions, 0 deletions