feat: add StructContainerTreeView, model Validator as StructContainerType

Add StructContainerTreeView — a tree view for fixed-size containers
that stores the value as a flat Zig struct. Fields are read and written
in O(1) via direct struct access. Dirty fields are tracked with a
StaticBitSet. On commit, only changed fields are spliced into the
merkle tree via setNodesAtDepth. The pool has no special node types —
this is purely a view-level optimization.

- Add StructContainerTreeView with StaticBitSet dirty tracking
- Parameterize FixedContainerType with TreeViewType enum
- StructContainerType selects StructContainerTreeView via the enum
- Model phase0 Validator as StructContainerType
- set() skips write and dirty-marking when value is unchanged
- serializeIntoBytes/toValue read from the struct directly (no commit needed)
- commit has clear ownership phases with correct errdefer cleanup

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wemeetagain

src/ssz/root.zig

@@ -33,6 +33,7 @@ pub const VariableVectorType = types.VariableVectorType;
 
 pub const FixedContainerType = types.FixedContainerType;
 pub const VariableContainerType = types.VariableContainerType;
+pub const StructContainerType = types.StructContainerType;
 
 pub const getPathGindex = types.getPathGindex;
 
@@ -42,6 +43,7 @@ pub const HasherData = hasher.HasherData;
 
 const tree_view = @import("tree_view/root.zig");
 pub const ContainerTreeView = tree_view.ContainerTreeView;
+pub const StructContainerTreeView = tree_view.StructContainerTreeView;
 pub const ArrayBasicTreeView = tree_view.ArrayBasicTreeView;
 pub const ArrayCompositeTreeView = tree_view.ArrayCompositeTreeView;
 pub const ListBasicTreeView = tree_view.ListBasicTreeView;