chore: npm publish-prep (os win32, alpha.3) + doc honesty fixes

[ipfizz]

Makes bunmaska publish-ready and closes the package/site honesty gaps the readiness audit flagged.

• os now includes win32 - without this, npm i bunmaska hits EBADPLATFORM on Windows while the site says "Windows ships." (the hard blocker)
• Bump to 0.1.0-alpha.3 so the registry's first artifact is Windows-correct; homepage → bunmaska.org; +windows/win32/wincairo keywords.
• Fixed the live ARM64 contradiction on the roadmap (meta + a card said "ARM64 + x64" while the body says x64-only) → "x64 today, ARM64 next."
• Fixed the stale building.md ("Windows not available yet") → the real --target windows + WinCairo-engine story.

Left intentionally as-is: files/exports/.ts-source/engines.bun (correct for a Bun-only ESM package); no sideEffects:false (would break app.whenReady()'s bootstrap import). Validate green (1589 tests); site builds.

🤖 Generated with Claude Code

[ipfizz]

Superseded by #8, which carries this PR's two commits (os: win32, 0.1.0-alpha.3, homepage, ARM64/Windows doc fixes) on top of the repo restructure (framework → root, one README, docs → website/).