docs: document PrimitiveNode copy/paste semantics and widgets_values loss (#9119)

## Summary

Documents the PrimitiveNode copy/paste bug mechanism and connection
lifecycle semantics in `WIDGET_SERIALIZATION.md`. This is tribal
knowledge from debugging
[#1757](https://github.com/Comfy-Org/ComfyUI_frontend/issues/1757) and
the related [Slack
discussion](https://comfy-organization.slack.com/archives/C09AQRB49QX/p1771806268469089).

## What's documented

- **The clone→serialize gap**: `_serializeItems()` calls
`item.clone()?.serialize()`. The clone has no `this.widgets`
(PrimitiveNode creates them on connection), so `serialize()` silently
drops `widgets_values`.
- **Why seed survives but control_after_generate doesn't**: Primary
widget value is copied from the target on reconnect; secondary widgets
read from `this.widgets_values` which was lost.
- **Current vs. proposed lifecycle**: Empty-on-copy → morph-on-connect
(current) vs. clone-configured-instance → empty-on-disconnect
(proposed).
- **Design considerations**: `input.widget` override flexibility,
deserialization ordering, and the minimal `serialize()` override fix.

## Related

- Issue: #1757
- Fix PR: #8938
- Companion: #9102 (initial WIDGET_SERIALIZATION.md), #9105 (type/JSDoc
improvements)
- Notion: COM-15282

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9119-docs-document-PrimitiveNode-copy-paste-semantics-and-widgets_values-loss-3106d73d3650816ba7f7d9e6f3bb3868)
by [Unito](https://www.unito.io)

docs/adr/0006-primitive-node-copy-paste-lifecycle.md

@@ -0,0 +1,45 @@
+# 6. PrimitiveNode Copy/Paste Lifecycle
+
+Date: 2026-02-22
+
+## Status
+
+Proposed
+
+## Context
+
+PrimitiveNode creates widgets dynamically on connection. When copied, the clone has no `this.widgets`, so `LGraphNode.serialize()` drops `widgets_values` from the clipboard data. This causes secondary widget values (e.g., `control_after_generate`) to be lost on paste. See [WIDGET_SERIALIZATION.md](../WIDGET_SERIALIZATION.md#primitiveno-and-copypaste) for the full mechanism.
+
+Related: [#1757](https://github.com/Comfy-Org/ComfyUI_frontend/issues/1757), [#8938](https://github.com/Comfy-Org/ComfyUI_frontend/pull/8938)
+
+## Options
+
+### A. Minimal fix: override `serialize()` on PrimitiveNode
+
+Override `serialize()` to fall back to `this.widgets_values` (set during `configure()`) when the base implementation omits it due to missing `this.widgets`.
+
+- **Pro**: No change to connection lifecycle semantics. Lowest risk.
+- **Pro**: Doesn't affect workflow save/load (which already works via `onAfterGraphConfigured`).
+- **Con**: Doesn't address the deeper design issue — primitives are still empty on copy.
+
+### B. Clone-configured-instance lifecycle
+
+On copy, the primitive is a clone of the configured instance (with widgets intact). On disconnect or paste without connections, it returns to empty state.
+
+- **Pro**: Copy→serialize captures `widgets_values` correctly. Matches OOP expectations.
+- **Pro**: Secondary widget state survives round-trips without special-casing.
+- **Con**: `input.widget[CONFIG]` allows extensions to make PrimitiveNode create a _different_ widget than the target. Widget config is derived at connection time, not stored, so cloning the configured state may not be faithful.
+- **Con**: Deserialization ordering — `configure()` runs before links are restored. PrimitiveNode needs links to know what widgets to create. `onAfterGraphConfigured()` handles this for workflow load, but copy/paste uses a different code path.
+- **Con**: Higher risk of regressions in extension compatibility.
+
+### C. Projection model (like Subgraph widgets)
+
+Primitives act as a synchronization mechanism — no own state, just a projection of the target widget's resolved value.
+
+- **Pro**: Cleanest conceptual model. Eliminates state duplication.
+- **Con**: Primitives can connect to multiple targets. Projection with multiple targets is ambiguous.
+- **Con**: Major architectural change with broad impact.
+
+## Decision
+
+Pending. Option A is the most pragmatic first step. Option B can be revisited after Option A ships and stabilizes.