On the live resolution path collectRootJsonlSessionIds already stat()s each root
jsonl for its mtime-window filter, then fileBelongsToTeam stat()ed the very same
file again for its cache validation -- two fs.stat syscalls (plus two Stats
allocations) per file, every poll. fileBelongsToTeam now takes an optional
precomputed stat and the mtime-filter caller passes the stat it already has, so the
file is statted once. Measured 20 files -> 20 stat calls on the mtime path (was ~40).
Using a single stat snapshot is also slightly more consistent than two reads that
could straddle a concurrent write. The other call site (subagent scan) passes no
stat and is unchanged (fileBelongsToTeam stats it itself). Adds a regression test
that a caller-supplied stat is the one recorded in the affinity cache.
776298b0e3