Implemented an overlap detection module in the postprocessing chain

README.md

@@ -148,10 +148,21 @@ The merged output uses the current seriatim envelope:
       "speaker": "Eric Rakestraw",
       "start": 1.25,
       "end": 3.5,
-      "text": "Hello there."
+      "text": "Hello there.",
+      "overlap_group_id": 1
     }
   ],
-  "overlap_groups": []
+  "overlap_groups": [
+    {
+      "id": 1,
+      "start": 1.25,
+      "end": 4.0,
+      "segments": ["eric.json#0", "mike.json#0"],
+      "speakers": ["Eric Rakestraw", "Mike Brown"],
+      "class": "unknown",
+      "resolution": "unresolved"
+    }
+  ]
 }
 ```
 
@@ -163,6 +174,21 @@ Segments are sorted deterministically by:
 
 Final segment IDs are assigned after sorting and start at `1`.
 
+## Overlap Detection
+
+The default postprocessing pipeline detects overlapping segment groups.
+
+Overlap behavior:
+
+- A strict timing overlap is required: `next.start < current_group_end`.
+- Segments that only touch at a boundary are not grouped.
+- Groups require at least two distinct speakers.
+- Transitive overlaps are grouped together.
+- Segments in detected groups receive `overlap_group_id`.
+- `overlap_groups[].segments` contains stable references in `source#source_segment_index` format.
+- `class` is currently `unknown`.
+- `resolution` is currently `unresolved`; overlap resolution is still a no-op.
+
 ## Autocorrect
 
 Autocorrect is included in the default postprocessing pipeline. If `--autocorrect` is omitted, the module leaves transcript text unchanged and records a skip event in the optional report.
@@ -202,5 +228,5 @@ Matching behavior:
 
 - Only JSON input is supported.
 - Word-level timing data is not preserved yet.
-- Overlap detection and overlap resolution are currently no-op modules.
+- Overlap resolution is currently a no-op module.
 - Coalescing and alternate output formats are not implemented yet.