fix: isolate document node snapshots

[soulteary]

Summary

• make Document.Nodes() return fully independent node and directive snapshots
• clone directive argument slices and inline-comment tokens instead of exposing internal mutable references
• add a regression test that mutates every exported nested value and verifies the document remains unchanged

Verification

• go test ./...
• go test -race ./...
• go vet ./...
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve non-nil empty argument slices

When AppendDirective or ReplaceDirective is called without arguments, syntheticDirective deliberately stores a non-nil empty Arguments slice, but cloning it with append([]Argument(nil), directive.Arguments...) collapses that value to nil. This makes Nodes() observably change the directive rather than merely copy it: nil checks change and JSON encoding produces null instead of the previous []. Preserve the source slice's nilness while cloning it.

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