Add platform guides and distro integration test matrix

[vmg-dev]

Summary

• add dedicated setup guides for macOS and Linux
• document how to choose --platform for conversions/tests
• turn integration tests into image-parameterized tests
• run integration tests in CI as a distro matrix (Alpine, Debian, Ubuntu, Fedora, Arch)

Validation

• bun run test
• bun run typecheck
• bun run build
• bun run test:integration (busybox)
• bun run test:integration with Alpine/Debian/Ubuntu/Fedora images on arm64 host
• bun run e2e:smoke

[vmg-dev]

Reviewed locally with gh CLI. Found one issue: in test/integration/oci2gondolin.integration.test.ts, assertProbeMissingFromBaseImage uses original GONDOLIN_GUEST_DIR when set, so the "base image" probe can run against custom assets and fail incorrectly. Consider unsetting GONDOLIN_GUEST_DIR for that check, then restoring it in finally.