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/metrics.py

@@ -0,0 +1,95 @@
+"""
+Pure diff functions for impact analysis.
+
+Implements FR-8.2: Structural diff-based impact measurement.
+
+Uses difflib.SequenceMatcher for computing change magnitude between
+old and new content versions.
+"""
+
+import difflib
+from typing import Optional
+
+
+def structural_diff_ratio(old_content: str, new_content: str) -> float:
+    """
+    Calculate structural similarity ratio between old and new content.
+
+    Uses SequenceMatcher to compute a ratio of matching blocks.
+    Returns the *change* ratio (1.0 - similarity), so higher values
+    mean more change.
+
+    Args:
+        old_content: Previous content
+        new_content: Current content
+
+    Returns:
+        Change ratio from 0.0 (identical) to 1.0 (completely different)
+    """
+    if old_content == new_content:
+        return 0.0
+    if not old_content and not new_content:
+        return 0.0
+    if not old_content or not new_content:
+        return 1.0
+
+    matcher = difflib.SequenceMatcher(None, old_content, new_content)
+    similarity = matcher.ratio()
+    return 1.0 - similarity
+
+
+def line_diff_ratio(old_content: str, new_content: str) -> float:
+    """
+    Calculate line-level change ratio between old and new content.
+
+    Splits content into lines and computes SequenceMatcher ratio
+    at the line level. Returns the change ratio.
+
+    Args:
+        old_content: Previous content
+        new_content: Current content
+
+    Returns:
+        Change ratio from 0.0 (identical) to 1.0 (completely different)
+    """
+    if old_content == new_content:
+        return 0.0
+    if not old_content and not new_content:
+        return 0.0
+    if not old_content or not new_content:
+        return 1.0
+
+    old_lines = old_content.splitlines()
+    new_lines = new_content.splitlines()
+    matcher = difflib.SequenceMatcher(None, old_lines, new_lines)
+    similarity = matcher.ratio()
+    return 1.0 - similarity
+
+
+def calculate_change_magnitude(
+    old_content: Optional[str],
+    new_content: Optional[str],
+    method: str = "structural",
+) -> float:
+    """
+    Calculate the magnitude of a change between two content versions.
+
+    This is the primary entry point for impact measurement (FR-8.2).
+
+    Args:
+        old_content: Previous content (None for created artifacts)
+        new_content: Current content (None for deleted artifacts)
+        method: Diff method to use ("structural" or "line")
+
+    Returns:
+        Change magnitude from 0.0 (no change) to 1.0 (complete change)
+    """
+    # Handle None cases (creation/deletion)
+    if old_content is None and new_content is None:
+        return 0.0
+    if old_content is None or new_content is None:
+        return 1.0
+
+    if method == "line":
+        return line_diff_ratio(old_content, new_content)
+    return structural_diff_ratio(old_content, new_content)