M4.82: PDF export writes top-level layers as PDF layers

Create PDF layers from top-level layers (document.exportPdf createLayers, the
Save PDF dialog's General option) now writes an optional content group per
top-level layer (template layers are left out): named as the layer, off when
the layer is hidden, /PrintState /OFF when its Print option is off, locked
when it is locked, listed top first, with a print auto-state (not under a
standard). The file reopens with those layers.

The PDF writer has no optional content, so each layer is drawn as a form of
its own (no transparency group, so it draws as in place) starting with a mark;
after writing, the uses of the layers' forms are wrapped in /OC ... BDC/EMC
and the catalog gets /OCProperties (crates/pdf/src/forms.rs, through the
shared patch writer and the PDF syntax reader of encrypt.rs). PDF 1.4 and a
knockout page group write plain page content with a warning; the dialog greys
the option at 1.4. The "not written yet" warning is gone.

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

docs/mcp.md

@@ -223,6 +223,9 @@ permissions; either password opens it (`document.open {password}`), and a passwo
 Pattern fills and strokes are written as their tiles clipped to the area they paint (a stroke's outline, with its
 dashes, caps, profile, arrowheads and alignment), and freeform gradients as an image of their colour field at the
 document's raster effects resolution, clipped the same way.
+`createLayers` writes each top-level layer (template layers are left out) as a PDF layer, an optional content group
+named as the layer: hidden layers are off, non-printing ones have `/PrintState /OFF` and locked ones are locked. It needs
+PDF 1.5 or later (at 1.4 it warns), and the file reopens with those layers.
 Images follow the `compression` settings of their kind (`color`, `gray`, or `mono` for black-and-white images): above
 `abovePpi` as placed they are resampled (`downsample`: `average`, `subsample` or `bicubic`) to `ppi`, and compressed
 with `zip`, `jpeg` (at `quality`; images with transparency stay lossless) or `auto` (JPEGs stay JPEG, the others