refactor(uffd): extract Prefault to its own file

[ValentaTomas]

Summary

• Pure file move: lift `Prefault` method and `directDataSource` helper out of `userfaultfd.go` into a new `prefault.go`.
• No behavior change.

Split out of #2419 to keep review surface minimal and prepare follow-ups (pageSize on struct, free page reporting).

Test plan

• `go build ./...` passes
• `go vet ./pkg/sandbox/uffd/...` clean

[cursor]

PR Summary

Low Risk
Low risk refactor that only moves Userfaultfd.Prefault and its helper type into a new file; main risk is unintended import/compile or package-level dependency issues rather than runtime behavior changes.

Overview
Refactors the userfaultfd package by moving Userfaultfd.Prefault and the directDataSource helper out of userfaultfd.go into a new prefault.go, with no intended behavior change.

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[claude]

LGTM — pure mechanical file move with no behavior changes.

Extended reasoning...

Overview\nThis PR moves the Prefault method and its directDataSource helper type verbatim from userfaultfd.go into a new prefault.go file. Two files are touched: the new file gains the moved code, and the old file has those lines deleted.\n\n### Security Risks\nNone. This is a pure refactor with no logic changes. No auth, crypto, or permission-sensitive code is involved.\n\n### Level of Scrutiny\nVery low. The diff is a clean cut-and-paste with no modifications to the moved code. The only identified issue is a pre-existing misleading error message ("less than" when the check is inequality), which has no functional impact and predates this PR.\n\n### Other Factors\nThe pre-existing error message nit is captured as an inline comment. It is minor (diagnostic text only, no runtime behavior change) and does not warrant blocking this refactor. Build and vet checks pass per the PR description.