feat(search): per-field case-insensitive search via _lowercase siblings

Keyword and path fields always index their case-preserved base and, when CaseInsensitive is set, an additional <field>_lowercase sibling used only for matching. The KQL lowering marks a restriction case-insensitive; each backend searches the sibling and lowercases the query value the same way the sibling is precomputed at index time (Go strings.ToLower on both sides, so non-ASCII stays consistent).

Search always returns the case-preserved base, so the sibling never has to be read back. In bleve it is indexed but not stored, kept out of _all, and without doc values. In OpenSearch it deliberately stays in _source: excluding it would make every update-by-query script rebuild all siblings from the document via painless toLowerCase, which lowercases differently than Go and would drift from the query side. Keeping it in _source avoids that, and a lowercased copy of a name or path is negligible disk in a cluster.

The OpenSearch move script keeps the base and its sibling in sync by swapping the moved prefix in Path_lowercase and setting Name_lowercase from Go-lowercased params, so case-insensitive search still finds a file after it moves (previously the sibling went stale). bleve re-indexes the whole document on move/delete/restore, so its siblings stay fresh for free.

This also repairs OpenSearch path search (the query value was no longer folded to lowercase, so path:<Foo> returned nothing) and makes bleve path queries match a folder and its descendants like OpenSearch's path_hierarchy. The Path base stays case-preserved so the move/delete descendant update (an exact TermQuery on Path) matches mixed-case folders.

pkg/ast/ast.go

@@ -44,6 +44,9 @@ type StringNode struct {
 	Key   string
 	Value string
 	Exact bool
+	// CaseInsensitive marks a case-insensitive restriction; set by the search
+	// lowering pass, a backend routes it to the field's lowercased form.
+	CaseInsensitive bool
 }
 
 // BooleanNode represents a bool value