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).
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