M11: say redaction and export don't read Group 4 yet; Reduce File Size's descriptions

- Redaction doesn't decode CCITT, so a mark over part of an image the
  optimizer stored as Group 4 removes the whole image (nothing leaks),
  and Export ▸ Images refuses it. The optimize README and the
  mono-compression parity note say so. Later PRs of this stack add both
  (#4335 export, #4384 redaction).
- doc_reduce's description, Engine::reduced_bytes and docs/cli-batch.md
  now list the black-and-white step; docs/target-app-parity.md's
  optimize row gives the measured counts and no longer lists CCITT as
  missing; the M11 milestone row lists it as done.

From reviews with the pdfcraft-review skill.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P6yUtvWGPsJpMLyh6pxSXG

docs/cli-batch.md

@@ -27,7 +27,8 @@ new engine importers become available here automatically. Without `convert`,
 only `.pdf` inputs are processed. Unsupported extensions are reported as skipped.
 
 - `optimize` uses Reduce File Size settings: JPEG quality 60, resampling to 150 ppi
-  above 225 ppi, plus the existing document cleanup. `--ppi` (9–1600) sets the target
+  above 225 ppi (black-and-white images stored as CCITT Group 4 when smaller), plus the
+  existing document cleanup. `--ppi` (9–1600) sets the target
   and a threshold of 1.5 times that value; `--quality` (1–100) sets JPEG quality.
   These settings can lose image detail. A candidate that is not smaller retains
   the bytes entering that step. This is not an images-only or lossless preset.