fix(engine): stop the compressors rewriting line endings and eliding real changes

splitLines/joinLines keep every line's own CR attached, so a document with
mixed endings survives a round trip; the old "one CRLF anywhere picks the
separator" rule rewrote whole files. isChangedDiffLine no longer classifies
a change whose content starts with '-'/'+' (or a bare blank-line change) as
context, and splitTextSections keeps a fenced code block as one unit instead
of splitting and TrimSpace-ing it into pieces.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SqmbqWDtNexzDfH5V3BMRz