ShellFileOperations.read_file previously ran sed -n '{off},{end}p' bare, so
a file with one pathological line (e.g. a 50MB+ minified bundle on a single
line) shipped the entire line across the exec transport before Python's
per-line clamp (_add_line_numbers, MAX_LINE_LENGTH=2000) could trim it.
read_file now pipes through 'cut -b1-{4*max_line_length+1}' so the shell
bounds every line to 8001 bytes before the bytes ever reach Python.
UTF-8 finding: GNU 'cut -c' is byte-based despite its name (verified:
cutting a line of 2-byte 'é' at -c8004 splits a codepoint, leaving a bare
0xC3 lead byte). The transport decodes with errors='replace', so a split
codepoint becomes U+FFFD rather than raising — but a clamp of
max_line_length+1 BYTES would deliver under max_line_length CHARS for
multibyte text, so the Python clamp would never fire and truncation would
be silent. Using 4*max_line_length+1 bytes (UTF-8 max 4 bytes/codepoint)
guarantees any line longer than max_line_length chars still decodes to
more than max_line_length chars, so len(line) > max_line_length always
triggers the existing '... [truncated]' suffix, and any boundary U+FFFD
lands past char max_line_length where the clamp removes it — verified
empirically with fixtures ('é'*4001 splits at the byte boundary yet the
result contains no U+FFFD and ends with the truncated suffix). 'cut -b'
is used explicitly to document the byte semantics.
cut (unlike sed -n p) always newline-terminates its output, which would
grow a phantom empty final line on files without a trailing newline; the
final-page path now probes the last byte (tail -c 1 | wc -l) and strips
the artifact.
read_file_raw is untouched: it is documented as no-per-line-truncation.
Benchmark (50MB single-line fixture, /usr/bin/time -v, median of 3):
before: 191.1 MB peak RSS, 1260 ms wall
after: 97.8 MB peak RSS, 490 ms wall
Correctness identical in both arms: monster line returns the clamped
2000-char form + '... [truncated]', offset=2 returns the trailing normal
lines intact.
Tests: 153 passed, 0 failed, 4 skipped across the file-ops suites plus a
new tests/tools/test_read_shell_line_clamp.py pinning the monster-line
clamp, offset-past-monster reads, no-trailing-newline preservation, both
UTF-8 boundary cases, and read_file_raw's exemption. Two existing mocks
asserting the exact sed command string were updated for the pipeline.
