The locator's per-root fast path previously only probed the root's direct
children, so drops like ~/Downloads/dump2 11/iotclaw.ndjson (a file two
levels deep inside a huge Downloads tree) fell through to the bounded
recursive walk, which exhausted its 20,000-entry budget inside unrelated
large directories and returned not-found (toast: 未能定位原始路径).

Replace the direct-child probe with a depth-1..3 shallow scan: the root's
direct child, direct children of each direct subdirectory, and direct
children of their subdirectories. Depths 1-2 are blind stats; only depth 3
expands the direct subdirectories (capped by SHALLOW_MAX_DIRS). The
bounded recursive walk stays as the fallback for deeper files and for
collecting every same-named candidate.

Measured on ~/Downloads (526k entries): the fast path covers all 15
same-named candidates in ~40ms and resolves the reported file in ~700ms
including the Spotlight probe, instead of 380ms + not-found.