docs(claude): document cross-platform validation (macOS host, Linux via Docker, Windows VM)

Rename the Windows section to "Cross-platform validation": macOS is the dev
machine / default npm test target; on Linux use Docker (node:22-bookworm,
`docker run --rm --init` so process-lifecycle tests don't false-fail on
un-reaped zombies; count inotify via /proc/<pid>/fdinfo); Windows VM kept.
Also record the pre-existing mcp-initialize/mcp-roots EPERM failures on Windows.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>