M2.6: JBIG2 decoding in pdfcraft-filters; Document::decoded_image reads JBIG2 images

decode_jbig2 sits next to decode_ccitt, through hayro-jbig2 as Export ▸ Images
used it, and shares CCITT's row sink. The JBIG2 page has to be the image's size:
PdfCraft's renderer draws the whole page into the image (black box: a 4-row page
behind /Height 2 fills the image box with all four rows), so the rows of a taller
page aren't the image's. The decoder decodes a page whole, so the page is checked
before it is decoded: that size, within the caller's bound as rows of one bit, and
at most 2^28 pixels (MAX_JBIG2_PIXELS; the decoder keeps a bit a pixel, about 34 MB
there).

Document::decoded_image_within decodes a JBIG2 image as it does a fax image: the
filters before it undone first, then the decoder with the /JBIG2Globals of the
image's own /DecodeParms entry (/Crypt counts in the position; fax_params and the
globals now share last_filter_parms). Globals have a limit of their own,
MAX_JBIG2_GLOBALS (32 MiB, moved from Export), and globals that can't be read are an
error rather than ignored.

Export ▸ Images reads JBIG2 through it, so its own decoder, row sink and DecodeParms
reader go, and hayro-jbig2 leaves pdfcraft-create. JBIG2 behind another filter now
exports, and so copies (Copy Image uses the same function); a page taller than the
image, which Export cut to the image's rows, now skips the image, as one of another
width did. doc_export_all_images' description no longer lists JBIG2 inside other
filters as skipped.

The optimizer's monochrome step read images through decoded_image_within and
relied on JBIG2 failing there: it now leaves JBIG2 images alone explicitly, as
before (a resampled one would come out Flate under Retain), and the README and
parity note list them as not yet handled.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/optimize/src/images.rs

@@ -802,6 +802,9 @@ fn one_bit(doc: &Document, s: &Stream, pack: bool, defaults: bool) -> Option<(Ve
         let len = stride.checked_mul(hu)?;
         let max = match s.filters().last() {
             Some((pdfcraft_filters::Filter::CcittFax, _)) => wu.checked_mul(hu)?.checked_add(hu.checked_mul(4)?)?.checked_add(4096)?,
+            // `decoded_image_within` reads JBIG2 too, but not for this yet: a resampled JBIG2 image
+            // would come out Flate under Retain, and nothing weighs JBIG2 again against it.
+            Some((pdfcraft_filters::Filter::Jbig2, _)) => return None,
             _ => allowance(len, h)?,
         };
         let mut data = doc.decoded_image_within(s, max).ok()?;