fix: name the flag in the preflight notice and tidy docs

Commit

Andrea Nodari2w ago
  • The notice names --target-max-pack-bytes again: CLI users are most of its readers, and library callers map it to TargetMaxPackBytes.
  • docs/usage.md says again how a reported size of 0 is treated.
  • The sanitize package doc separates raw C1 bytes, which UTF-8 terminals ignore, from UTF-8-encoded C1 characters, which Line strips.
  • TestLineKeepsMessageOnOneSafeLine no longer splits the Writer test from its doc comment.

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_015oS6W9qjGMVnrTz5r8EwDL Entire-Checkpoint: 01M5FEE4HNPGWQTD9XCADVA9ZC