Add source attachment metadata compatibility

examples/source-attachments/epub3-attachments.normalized.yaml

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+schema_version: markitect.source.v1
+document_id: source.epub3:fixture
+adapter:
+  id: source.epub3
+  version: "1"
+attachments:
+  - uri: fixture.epub!/EPUB/images/chart.png
+    path: EPUB/images/chart.png
+    name: chart.png
+    media_type: image/png
+    extension: .png
+    size: 15
+    digest: sha256:example-chart
+    metadata:
+      schema_version: markitect-filter.source-attachment.v1
+      source_adapter: source.epub3
+      source_role: image
+      package_path: EPUB/images/chart.png
+      href: images/chart.png
+      manifest_id: chart
+      render_manifest_compatible: true
+  - uri: fixture.epub!/EPUB/styles/book.css
+    path: EPUB/styles/book.css
+    name: book.css
+    media_type: text/css
+    extension: .css
+    size: 22
+    digest: sha256:example-css
+    metadata:
+      schema_version: markitect-filter.source-attachment.v1
+      source_adapter: source.epub3
+      source_role: stylesheet
+      package_path: EPUB/styles/book.css
+      href: styles/book.css
+      manifest_id: style
+      render_manifest_compatible: true
+render_asset_manifest_handoff:
+  compatible_schema: markitect.render.reference.v1
+  core_asset_copying: false
+  renderer_required: false

examples/source-attachments/pdf-attachments.normalized.yaml

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+schema_version: markitect.source.v1
+document_id: source.pdf:fixture
+adapter:
+  id: source.pdf
+  version: "1"
+attachments:
+  - uri: fixture.pdf!/embedded/attachment.txt
+    path: embedded/attachment.txt
+    name: attachment.txt
+    media_type: text/plain
+    extension: .txt
+    size: 13
+    digest: sha256:example-embedded-file
+    metadata:
+      schema_version: markitect-filter.source-attachment.v1
+      source_adapter: source.pdf
+      source_role: embedded-file
+      package_path: embedded/attachment.txt
+      pdf_object: 8
+      embedded_file_name: attachment.txt
+      render_manifest_compatible: true
+  - uri: fixture.pdf#page-0001/image-signal
+    path: page-0001/image-signal
+    name: image-signal
+    media_type: application/x.markitect-pdf-image-signal
+    digest: sha256:example-image-signal
+    metadata:
+      schema_version: markitect-filter.source-attachment.v1
+      source_adapter: source.pdf
+      source_role: image-signal
+      signal_only: true
+      page: 1
+      pdf_object: 3
+      image_objects:
+        - 5
+      render_manifest_compatible: true
+render_asset_manifest_handoff:
+  compatible_schema: markitect.render.reference.v1
+  core_asset_copying: false
+  renderer_required: false