feat(cli): implement missing output element types and handles

Implement LiveMaterializationStrategy as the third materialization
strategy type (alongside sequential_buffer and accumulate) per spec
§26456-26492.  The live strategy renders element updates in-place at
~15 fps by tracking a dirty-element set and coalescing updates into
frame redraws.

RichMaterializer now extends LiveMaterializationStrategy instead of
_BaseBufferStrategy, enabling supports_incremental_updates=True.
Element rendering still delegates to the colour renderer for visual
formatting, maintaining backward compatibility.

Changes:
- Add _LiveMaterializationStrategy class with frame-rate throttling,
  dirty-element tracking, and per-frame composition in declaration
  order
- Update RichMaterializer to extend _LiveMaterializationStrategy
- Export LiveMaterializationStrategy from materializers.py and
  __init__.py
- Update SD-2 and SD-7 documentation in __init__.py
- Add vulture whitelist entry for LiveMaterializationStrategy
- Add 5 Behave scenarios covering live strategy rendering, incremental
  update support, dirty-element coalescing, all-element-type rendering,
  and frame rate validation
- Add step definitions for the new scenarios

ISSUES CLOSED: #903

features/output_rendering.feature

@@ -2186,3 +2186,51 @@ Feature: Output Rendering Framework
     Given an OutputSession with format "plain"
     When I create a tree handle with root label "Root"
     Then querying a path with wrong root returns None
+
+  # ================================================================
+  # LiveMaterializationStrategy
+  # ================================================================
+
+  Scenario: LiveMaterializationStrategy renders element on creation
+    Given an OutputSession with format "rich" using RichMaterializer
+    When I create a panel handle with title "Live Panel"
+    And I set entry "Key" to "Val"
+    And I close the panel handle
+    And the session is closed
+    Then the rich output contains "Live Panel"
+    And the rich output contains "Key"
+
+  Scenario: LiveMaterializationStrategy supports incremental updates
+    Given a RichMaterializer instance
+    Then the rich materializer supports incremental updates
+    And the rich materializer strategy name is "rich"
+
+  Scenario: LiveMaterializationStrategy coalesces dirty elements
+    Given an OutputSession with format "rich" using RichMaterializer
+    When I create a text handle with content "initial"
+    And I append text " more"
+    And I close the text handle
+    And the session is closed
+    Then the rich output contains "initial more"
+
+  Scenario: LiveMaterializationStrategy renders all element types
+    Given an OutputSession with format "rich" using RichMaterializer
+    When I create a tree handle with root label "Root"
+    And I add a tree child "child" under "Root"
+    And I close the tree handle
+    And I create a code handle with content "x = 1" and language "python"
+    And I close the code handle
+    And I create a diff handle with file_a "a.py" and file_b "b.py"
+    And I add a diff hunk with header "@@ -1 +1 @@" and context line "ctx" and add line "new"
+    And I close the diff handle
+    And I create a separator with style "line"
+    And I create an action hint with commands "deploy"
+    And the session is closed
+    Then the rich output contains "Root"
+    And the rich output contains "x = 1"
+    And the rich output contains "a.py"
+    And the rich output contains "deploy"
+
+  Scenario: LiveMaterializationStrategy frame rate is approximately 15 fps
+    Given a RichMaterializer instance
+    Then the live strategy frame rate is 15.0