feat: add rooted io/fs adapter

[iahsanGill]

Adds a RootedFS compatibility layer that exposes a rooted kyaml/filesys.FileSystem through the standard io/fs read interfaces. On-disk filesystems use os.Root; in-memory and other implementations use the same rooted relative-path contract through an adapter.

This keeps the existing public FileSystem API unchanged and provides a bounded migration path for #6220.

Tests cover io/fs conformance, in-memory and on-disk filesystems, sub-filesystems, invalid paths, and on-disk symlink containment.

Tests:

• go test -race ./filesys
• go vet ./filesys
• go test ./...
• repository lint, license, module-tidiness, and API compatibility checks

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kubernetes-prow]

Hi @iahsanGill. Thanks for your PR.

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