feat: preserve explicit YAML document start markers

[aausch]

What this does

Adds PreserveDocStart to kio.ByteReader and kio.LocalPackageReader, mirroring the existing PreserveSeqIndent option. When set, a leading --- is recorded in the internal.config.kubernetes.io/docstart annotation at read time and re-emitted by ByteWriter for the first document of the stream. It is off by default, so no existing output changes.

Why

kyaml drops a leading --- on round trip. The reader consumes the marker, and the encoder only writes a separator ahead of the second and later documents, so a file that began with an explicit document start comes back without one.

This is visible to anyone who patches manifests in place with kyaml. The case I hit is Flux's image-automation-controller, which rewrites an image tag in a manifest and commits the result: every such commit also silently strips the ---, which then fails YAML linters that require the marker. It was reported as fluxcd/flux2#4435 and closed with "that's what kustomize kyaml does when it patches the YAML, nothing we can do about it" — correct as of today, hence this option. The companion change is fluxcd/image-automation-controller#1093, which sets the flag and is blocked on a kyaml release carrying this.

Implementation notes

• Only the first document of a stream needs the annotation. splitDocuments splits on a separator preceded by a newline, so a marker only survives in the original text of the first document; every later document is written with a separator regardless.
• The marker is written directly to the writer ahead of the encoder, which has emitted nothing at that point, so the two writes stay ordered.
• Nodes wrapped in a List/ResourceList are a single document and carry no marker of their own, so the wrapping path is untouched.
• kioutil.DocStartAnnotation is added to api/resource.BuildAnnotations alongside SeqIndentAnnotation.

Tests

• TestByteReadWriter_PreserveDocStart — marker preserved, no marker invented, marker dropped when the option is off, and both multi-document orderings round-trip unchanged.
• TestByteReadWriter_PreserveDocStartOmitReaderAnnotations — the option conflicts with OmitReaderAnnotations, matching how PreserveSeqIndent behaves.
• TestLocalPackageReader_Read_PreserveDocStart — the annotation reaches only the resource that was preceded by a marker.
• go test ./... passes in kyaml and in api.

This PR was written in part with the assistance of generative AI.

/kind feature

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: aausch / name: Alex Ausch (d531936)

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