docs: group hierarchies and fold lines, with the evidence for each rival (improvise-felo.6)

The harness fixtures open their groups explicitly now that groups start closed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

harness/fixtures/grouped.improv

@@ -11,6 +11,8 @@ _Measure: page, Revenue
 _Index: none
 _Dim: none
 collapsed: Region/Upland
+expanded(1): Month
+expanded(1): Region
 format: ,.0
 
 ## Category: Region

harness/fixtures/tall.improv

@@ -8,6 +8,7 @@ _Dim: none
 _Measure: page
 Region: row
 format: ,.0
+expanded(1): Region
 
 ## View: Tall
 _Index: none
@@ -15,6 +16,7 @@ _Dim: none
 _Measure: column
 Region: row
 format: ,.0
+expanded(1): Region
 
 ## Category: _Measure
 - Revenue, Cost