Release 0.13.0-alpha.2 standalone PDF page extraction

[MiLattanzio]

Summary

• add a bounded managed standalone-page PDF writer and public page/document extraction APIs
• add the poppler-net separate command with deterministic collision-safe names
• expose autonomous page PDF and all-pages ZIP downloads in the WebAssembly playground
• document preservation, encryption and safety policies
• extend package smoke coverage and the Ubuntu/Windows/macOS CLI consumer matrix

Qualification

• Release build: 0 warnings, 0 errors
• NUnitLite: 309/309 tests passed
• managed-only verification passed
• library and CLI NuGet package verification passed
• clean package consumers passed for net8.0 and net10.0
• packaged dotnet tool install, HTML export, page separation and collision-safe naming passed
• real-world three-page extraction reopened and rendered pixel-identically to the source pages
• local WebAssembly playground loaded as 0.13.0-alpha.2 with the new PDF exports

Closes #29