fileBelongsToTeam streamed the head window via createReadStream + readline. readline's
line iterator runs an expensive Unicode line-break regex and stream/string-decoder
machinery per chunk, which showed up as a top main-thread cost during launch (the line-
split regex alone was ~5.7% in the warm launch profile).
Replace it with a bounded chunked fs.read + a plain '\n' split. JSONL is strictly
newline-delimited and each line is trim()'d (so a trailing CR from CRLF is dropped),
so a '\n' split is cheaper and more correct (it will not split on a bare CR or a
Unicode line/paragraph separator inside a JSON string value, which readline would). A
StringDecoder preserves multi-byte UTF-8 sequences that straddle a chunk boundary.
Byte-identical semantics to the old loop: inspect up to TEAM_AFFINITY_SCAN_LINES
non-empty lines, first match wins via early break, and a final line is honored even
without a trailing newline. Reads in 64KB chunks so a team decided in its first lines
is not penalized by a huge file. Adds tests for CRLF endings + no-trailing-newline,
a multi-byte char straddling the 64KB boundary, and the 40-line window bound (21 pass).
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.
fileBelongsToTeam only cached POSITIVE affinity durably; a negative verdict was
re-decided on any change, so during a launch every non-matching transcript in the
project dir that grew (mtime+size change from an active session) was re-streamed
(createReadStream+readline) and re-parsed (up to 40 head lines) on every bootstrap
poll. A live atlas-hq-5 launch profile put this whole subsystem (readline streaming
+ fileBelongsToTeam + line/team matching) at ~31% of main-thread JS, the single
largest launch cost.
A team's first 40 head lines are immutable for an append-only transcript, so a
`false` decided from a FULL inspected window (>= TEAM_AFFINITY_SCAN_LINES) stays
valid while the file only grows. Track headWindowFull on the cache entry and short-
circuit such negatives the same way positives are short-circuited (size >= cached).
Short files (partial window) are still re-scanned on growth, so a team mention that
later lands inside the head window is still detected. A shrink/rewrite (size <
cached) forces a re-scan, identical to the positive path.
Behavior-preserving for affinity correctness (no new false negatives); only removes
redundant re-streams. Adds regression tests for both the durable-negative and the
short-file-flips-to-true cases.
