Support the UseAttachments page mode, and write /PageMode once (mirrors mpdf/mpdf#2142)

[jakejackson1]

Summary

This mirrors mpdf#2142, so the fork carries the feature while the upstream PR sits open.

DisplayPreferences gains UseAttachments, which asks the reader to open its attachment pane — the counterpart to SetAssociatedFiles(), which already embeds the files but gives no way to say "show them".

writeCatalog() had three separate places writing /PageMode: the outline pane implied by having bookmarks, FullScreen from DisplayPreferences, and UseOC for the layer pane. A document with bookmarks, layers and open_layer_pane set already emitted the key three times in one dictionary, which a PDF dictionary cannot hold. Rather than add a fourth, the choice now happens once.

Try it

$mpdf = new \Mpdf\Mpdf();
$mpdf->SetAssociatedFiles([[
	'name'        => 'invoice.xml',
	'mime'        => 'text/xml',
	'description' => 'Invoice data',
	'AFRelationship' => 'Alternative',
	'path'        => __DIR__ . '/invoice.xml',
]]);
$mpdf->SetDisplayPreferences('UseAttachments');
$mpdf->WriteHTML('<p>Invoice</p>');
$mpdf->Output();

The catalog carries /PageMode /UseAttachments, and the reader opens with the attachment pane showing.

Test plan

• composer test — 1054 tests, 2541 assertions, green (gravitypdf is at 1048).
• composer cs — clean.
• PHPStan — same 33 findings as gravitypdf, none new.
• tests/Mpdf/PageModeTest.php — three of its six cases fail on gravitypdf: the two for the new keyword, and testLayerPaneWinsOverEverythingElse, which catches the existing duplicate-key bug.

More info

Where it differs from upstream

Upstream	Here
Adds a fourth /PageMode writer next to the FullScreen one	Resolves the mode once, in MetadataWriter::getPageMode()
Space-indented in a tab-indented file (composer cs rejects it)	—
No test	Adds six

Precedence

Readers resolve a repeated dictionary key by taking the last one written, so the order below is the one documents get today, with the new keyword slotted in among the explicit requests:

1. UseOC — open_layer_pane with layers present
2. FullScreen — DisplayPreferences
3. UseAttachments — DisplayPreferences
4. UseOutlines — the only one the document never asks for; it follows from having any bookmark at all

So nothing that resolved to a given mode before resolves differently now.

PDF version

/PageMode /UseAttachments is PDF 1.6, and pdf_version defaults to 1.4. That matches how /PrintScaling (1.6) and /Duplex (1.7) are already handled here — written with a comment noting the version, no header bump — so this follows suit rather than changing when mPDF advertises a higher version.

Also touched

SetDisplayPreferences() carried a comment listing the accepted keywords that had drifted — no FullScreen, no NoPrintScaling. It now lists what the writer actually reads.