M4.86–M4.87: PDF thumbnails, fast web view and PDF 1.3 (M4 done)

[bflatastic]

PDF page thumbnails, fast web view (linearised files) and PDF 1.3 compatibility. This is the last M4 package; the ROADMAP now marks M4 done.

M4.86: thumbnails and fast web view

• thumbnails: true gives every page a /Thumb image, 106 px on its long side. PDF/X files get grey thumbnails, so X-1a stays CMYK-only.
• fastWebView: true writes a linearised file (pdf/src/linearize.rs, run from the post-write pipeline in post.rs).
  • Objects are reordered and renumbered before encryption and laid out after it, so encrypted files stay linearised.
  • If linearising fails, the file is written without it and a warning says so.
  • qpdf's linearisation check passed by hand on plain files and on 40-bit RC4, 128-bit RC4, AES-128 and AES-256 encrypted files. A hint-table checker in the tests catches wrong shared-object ids.
• With both options off, the output is byte-identical to before. Their "not embedded / not written yet" warnings are gone.

M4.87: PDF 1.3

• compatibility: "1.3" flattens a copy of the document with flattenerPreset (High Resolution by default; PDF presets store it).
  • Rasterised areas are clipped to their regions, so no soft masks remain.
  • The header and XMP say 1.3, and the writer refuses a file that still has transparency.
• Passwords at 1.3 use 40-bit RC4, the only encryption PDF 1.3 has.
• PDF/X-1a and X-3 now accept and default to 1.3. They share the 1.3 header code with this task, and they also take flattenerPreset (default unchanged).
• The Save PDF dialog lists PDF 1.3 and gains a Flattener preset dropdown.
• A test imports a 1.3 file, renders it, and checks it is within 2% of the canvas.

ROADMAP and README

• M4 is marked ✅ done. Still left: DWG (use DXF), placing PSDs as layers, overprint read back from PDF, and polish.
• "Flattener presets not applied in PDF/print" is removed from the M3, Appearance and Print rows.
• README: the status line lists the M4 formats, and the workspace list gains eps, cad and metafile.

Checks

• Rebased on main at 87df4a0 (M4.82–M4.83: PDF layers and overprint #123).
• cargo xtask ci passes all 7 steps (fmt, clippy, tests, assets, brands, layers, wasm), with 2,921 tests.
• Nothing dropped: compared with main, every test (2899 → 2914 by name), engine command, UI command, menu entry and MCP tool is still there.
• Tests changed because 1.3, thumbnails and fast web view now work:
  • pdf tests_settings: the camel-case test's bad version is now "1.2", and the warnings test no longer expects the thumbnails and fast web view warnings.
  • engine tests_pdf::bad_options_are_refused (1.2 row, new 1.3 rows) and options_not_applied_yet_and_document_features_warn.
  • tests_pdfpresets::saved_presets_apply_wherever_a_preset_is_named
  • tests_pdfx::choosing_a_pdfx_standard_… (1.4 → 1.3)
  • ui save_pdf: three dialog tests.
• docs/mcp.md code fences are balanced.

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