feat: align legacy operator and playout behavior

docs/UC1_TREE_EVENT_PARITY.md

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+# UC1 tree event parity
+
+Source evidence is the read-only original
+`C:\Users\MD\source\repos\MBN_STOCK_N\MBN_STOCK_N\Control\UC1.cs`:
+
+- `UC1_Load` checks `Expand`, selects the first rendered root, and calls
+  `EnsureVisible`.
+- `treeView_MouseDown`, `treeView_BeforeExpand`, and
+  `treeView_BeforeCollapse` prevent a double-click from also changing a
+  root's expanded state.
+- `treeView_DoubleClick` activates either the selected leaf or every direct
+  child of the selected root.
+- `chE_CheckedChanged` expands or collapses every root. Collapsing does not
+  clear `SelectedNode`.
+
+The WebView tree therefore owns only presentation state (open roots, selected
+node, focused node, and scroll position). C# remains the authority for every
+leaf/root action. A root label click selects without expanding; only its
+disclosure glyph changes `open`. A root or leaf double-click emits one closed
+native intent and does not invoke the browser's whole-`summary` toggle.
+
+On each real tab activation/re-entry the presentation state is recreated like
+the original `UC1`: `Expand` is checked, all sections are open, and the first
+root is selected, focused, and scrolled into view. State-only renders within
+the same activation preserve per-section open state, selection, focus, and
+scroll position. Busy and fixed-section-batch states reject both actions and
+local expand/collapse changes.
+
+## Intentional defect exclusion
+
+The original `treeView_DoubleClick` reads `uc_3.FpSpread` in the parent-node
+path before checking which UC1 file is active. For UC1 instances constructed
+with `uc_3 == null`, that can throw `NullReferenceException`; the surrounding
+empty `catch` silently drops the operator action. This is an original defect,
+not observable intended behavior, so the migration does **not** reproduce it.
+The WebView sends only the selected root identity and C# validates/materializes
+the corresponding section without dereferencing an unrelated comparison
+control.