Packet D completed

Created docs/webapp-vault/30-source-walks/04-plugin-product-model.md.

Covered:

• plugin registration-to-render flow
• product route flow
• module federation exposure map
• Redux plugin component storage shape
• Pluggable selection rules
• Product vs ordinary plugin differences
• source table for required files
• known true facts and remaining uncertainty

Notes:

• channels/src/reducers/plugins/plugins.ts is absent in this checkout; reducer logic is in channels/src/reducers/plugins/index.ts.
• channels/src/types/plugins/index.ts is absent; plugin component/product state shape is in channels/src/types/store/plugins.ts.
• Product remote list is empty in this OSS source snapshot, so enterprise/runtime remote population remains outside this packet.

Verification:

• Markdown fence check passed: 8 fences, 4 Mermaid blocks, balanced.
• No app code edited.