b4363b9b36946b63017de1b6d8b8e924dbf23e56  Display PDFs in two-up when the iPhone Duo is partially folded

Classified as not security-relevant.
Triage note: Adds a two-up PDF display-mode feature for iOS (initialPDFDisplayMode plumbed through WebPageCreationParameters, WebPage(Proxy), SetInitialPDFDisplayMode message, division-region detection). Pure feature/plumbing with no memory-safety or logic bug fixed.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+1.2), area:Source/WTF/ (+1.0)