Reuse the writer's monotonic statement index to find the last physical line
of an assignment. Remove the full line-start scan performed for every key.
Preserve source edits, multiline values, BOM, CRLF and final-line handling.
Add LF/CRLF controls with adjacent assignments, multiline values, an empty
value and EOF without a newline. Validated 85 focused tests across six files,
check:fast, docs:check and discovery of all 1553 test files.
Twelve bounded public CLI uninstall tasks preserve equivalent replies,
configuration and original backups. Warm 10000-assignment elapsed medians
were 0.538/0.520 s and CPU 0.817/0.744 s before/after; small-input timings
were noisy and slower in the candidate. No general speedup, cold-cache,
real-client or analysis-provider claim. Heavy profiling remains stopped.
Fixes#4873
morluto