Appending via edit_lineno required the last line to be read, which made
writing to an empty file impossible (line 0 does not exist). An empty
file has nothing to clobber, so the append needs no prior read.
Wrong-target edits (the test_run_aside pass clobber, the edit_replace
mangling) were silent: the success message reported counts only, so the
model had no signal it replaced the wrong thing until later.
edit_lineno now reports the first removed line ("first: 'pass'") and
edit_replace reports the matched pattern ('old' -> 'new'), giving the
model an immediate self-correction signal.
The boundary overguard (read offset=50 shows lines 51+, then editing
line 50 fails) forced blind re-reads. The error now reports the read
window ("this turn read lines 51-86") and advises reading with a margin
around the target range; read_file's description says the same.
edit_lineno now rejects edits when the file changed since the numbered
read (mtime recorded at read time), so stale line numbers are caught
even when the mutation came from run_argv / an external process — the
class the explicit invalidations could never cover.
This lets write_file / edit_replace / edit_lineno drop their explicit
invalidate calls (the write itself changes the mtime, and the error is
now the accurate "changed since it was read" instead of a misleading
"not read"). copy/move keep explicit invalidation because shutil
preserves the source mtime, which the freshness check cannot see.
read_file(with_lineno=true) marked the whole requested slice before
applying the max_chars cap, so a truncated read silently unlocked every
line of the file for edit_lineno even though the model never saw them.
Mark only the complete numbered lines actually returned (excluding the
[Truncated] marker); the unseen tail stays unread.
NCBM