fix(examples): classify Drive aliases per listing id and bound seen

Aliasing is now decided per listing id: a mount is an alias once that
listing entry already has an inode, and add_shared_tree seeds that set
with the directly shared roots. A shortcut therefore no longer makes its
target look like an alias, and distinct shortcuts are each a first mount
even when they share a target. `ids` still maps shortcut target ids for
parent lookup and `ancestry` keeps both ids for cycle detection.

Stop recording cap-rejected (file, parent) pairs in `seen`. Once the
alias cap is reached, already-mounted files are skipped before their
parent list is built, and an unmounted listing entry is mounted once
under its first eligible parent. Only real mounts and permanent
cycle/depth skips are recorded. `tree_parents` sorts candidates so
cap/depth truncation is stable across runs.

Termination: every mount records its (listing id, parent) pair, each
listing entry is first-mounted at most once, and further mounts are
bounded by MAX_ALIAS_NODES, so the walk adds finitely many inodes. When a
pass adds nothing the loop stops, and cycle/depth pairs are recorded so
they are not retried.