feat(prompts): implement Phase 6 - Incremental Execution (FR-7, FR-8)

Add change detection, structural diff-based impact analysis,
configurable-depth incremental recomputation with circular suppression,
and impact debt tracking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

markitect/prompts/incremental/impact.py

@@ -0,0 +1,57 @@
+"""
+Impact analyzer for assessing change significance.
+
+Implements FR-8: Impact Analysis
+Stateless analyzer that computes change magnitude and decides
+whether recomputation is warranted based on configurable thresholds.
+"""
+
+from typing import Optional
+
+from markitect.prompts.incremental.metrics import calculate_change_magnitude
+from markitect.prompts.incremental.models import RecomputeConfig
+
+
+class ImpactAnalyzer:
+    """
+    Stateless impact analyzer for artifact changes.
+
+    Computes change magnitude using diff metrics and compares against
+    configurable thresholds to determine whether recomputation is needed.
+    """
+
+    def calculate_magnitude(
+        self,
+        old_content: Optional[str],
+        new_content: Optional[str],
+        method: str = "structural",
+    ) -> float:
+        """
+        Calculate the magnitude of a change.
+
+        Args:
+            old_content: Previous content (None for creation)
+            new_content: Current content (None for deletion)
+            method: Diff method ("structural" or "line")
+
+        Returns:
+            Change magnitude from 0.0 to 1.0
+        """
+        return calculate_change_magnitude(old_content, new_content, method)
+
+    def should_recompute(
+        self,
+        magnitude: float,
+        config: RecomputeConfig,
+    ) -> bool:
+        """
+        Determine whether a dependent should be recomputed based on magnitude.
+
+        Args:
+            magnitude: Change magnitude (0.0-1.0)
+            config: Recompute configuration with threshold
+
+        Returns:
+            True if magnitude meets or exceeds threshold
+        """
+        return magnitude >= config.impact_threshold