Session candidates labelled from projection checkpoints instead of a full
log fold per keystroke, with the uncheckpointed remainder folded once and
memoized while its log stays cold. The file index keeps answering while an
invalidated traversal rebuilds behind the caret, and its default exclusions
now cover build outputs so deep sources stay reachable.
Rows carry only what distinguishes them: a file names its parent directory,
a session names its workspace only when that workspace is not the current
one, and a drilled listing names none because its new breadcrumb does.
Resolves#3180
Related to #3154
Tianyi Cui