M10: Create ▸ Blank page uses the system region's paper (#5644) (#5651)

* M10: Create ▸ Blank page uses the system region's paper (#5644)

A new blank document was always US Letter. It now takes the paper of the
system region: Windows' own Region paper size (iPaperSize) first, then the
system locale on macOS or LC_ALL/LC_PAPER/LANG, A4 everywhere outside CLDR's
US-Letter regions, and Letter when the region can't be told (web).
The region parsing is shared with the Units preference.

* M10: the first locale variable set decides the paper; extensions are not regions (#5644)

LC_ALL, LC_PAPER and LANG are read in POSIX order and the first one set
decides, so LC_ALL=C no longer falls through to LANG's region; and a locale
tag's region is read from its base tag only, so en-US-u-nu-latn is US, not
NU (CodeRabbit on #5651).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbuSKAS2k5248LGFEapMUR

* M10: the blank page test covers A4 as well as Letter (#5644)

The app keeps the paper Create ▸ Blank page makes (blank_paper, the system
region's at start), so the test makes a blank page on A4 and on Letter
whatever the host's region (CodeRabbit on #5651).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbuSKAS2k5248LGFEapMUR

---------

Co-authored-by: Claude <noreply@anthropic.com>

parity/acrobat-features.toml

@@ -6988,8 +6988,8 @@ milestone = "M10"
 status = "shipped"
 commands = ["create.blank"]
 tools = ["doc_create"]
-evidence = ["crates/ui-egui/tests/it/create.rs::opening_images_and_text_converts_them_to_new_pdfs", "crates/create/src/tests.rs::blank_documents", "crates/automation/tests/it/automation.rs::creating_and_reducing_through_tools"]
-notes = ""
+evidence = ["crates/ui-egui/tests/it/create.rs::opening_images_and_text_converts_them_to_new_pdfs", "crates/ui-egui/tests/it/create.rs::a_blank_page_is_on_the_system_regions_paper", "crates/ui-egui/src/persist/paper.rs::region_decides_the_paper", "crates/create/src/tests.rs::blank_documents", "crates/automation/tests/it/automation.rs::creating_and_reducing_through_tools"]
+notes = "The page is the system region's paper (#5644): US Letter in the US, Canada, Mexico, the Philippines and the other CLDR Letter regions, A4 elsewhere; Windows' own Region paper size (iPaperSize) first, then the system locale (macOS) or LC_ALL/LC_PAPER/LANG; Letter when the region is unknown (web). The doc_create tool keeps Letter unless given width and height. Missing: a page size or count chooser like Acrobat's New Document dialog."
 
 [[feature]]
 id = "create.from-html"