Add resource identifier metadata marshaling

[ryanscottaudio]

Related Issue

#80

Description

This change adds the optional MarshalResourceIdentifierMeta interface. Related resources can implement it to add metadata to each resource identifier object.

For example:

type Layer struct {
	ID    string `jsonapi:"primary,layers"`
	Nodes []Node `jsonapi:"relationship" json:"nodes"`
}

type Node struct {
	ID    string `jsonapi:"primary,nodes"`
	Index int
}

func (n Node) MarshalResourceIdentifierMeta() any {
	return map[string]any{"index": n.Index}
}

jsonapi.Marshal(Layer{
	ID: "layer-a",
	Nodes: []Node{
		{ID: "node-a", Index: 0},
		{ID: "node-b", Index: 1},
	},
})

The relationship data now contains distinct metadata on each resource identifier:

{
  "data": {
    "type": "layers",
    "id": "layer-a",
    "relationships": {
      "nodes": {
        "data": [
          {"type": "nodes", "id": "node-a", "meta": {"index": 0}},
          {"type": "nodes", "id": "node-b", "meta": {"index": 1}}
        ]
      }
    }
  }
}

Previously, jsonapi:"meta" could only add metadata to the relationship object itself. Adding per-identifier metadata required modifying the encoded JSON after marshaling.

The existing relationship-object metadata behavior is unchanged. Identifier metadata uses the existing metadata validation, omits nil values, and preserves an empty metadata object.

Additional context

Tests cover to-one and to-many relationships, distinct metadata, nil and typed nil values, empty objects, invalid values, and both metadata locations together.

Local checks passed:

• go test ./...
• go test -race ./...
• go vet ./...

[ryanscottaudio]

/code blockers

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2026-09-15 17:24:36 UTC ℹ️ Devflow:

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2026-09-15 17:24:42 UTC ℹ️ Devflow: /code blockers

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[ryanscottaudio]

/remove

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2026-09-15 17:44:13 UTC ℹ️ Devflow: /remove

[ryanscottaudio]

/code blockers

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2026-09-15 17:47:15 UTC ℹ️ Devflow:

Checking merge blockers for #81...

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2026-09-15 17:47:22 UTC ℹ️ Devflow: /code blockers

No merge blockers detected.

[ryanscottaudio]

/merge

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PR already in the queue with status in_progress

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/remove

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[ryanscottaudio]

/merge -m squash

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The expected merge time in main is approximately 0s (p90).

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2026-09-15 19:51:42 UTC ❌ MergeQueue: The build pipeline has timeout

The merge request has been interrupted because the build 0 took longer than expected. The current limit for the base branch 'main' is 120 minutes.