fix: keep chunk details out of speaker headers

Render transcript-wide speaker numbers instead of exposing upload chunks in
reader-facing headers. Reconcile adjacent requests using at least two
unambiguous word-and-time matches in their shared audio, including when
provider speaker IDs change between requests. Reject conflicting mappings
and never treat matching numeric provider IDs as identity evidence.

Assign fresh IDs where continuity cannot be established and report that
uncertainty on stderr. Number only speakers retained after overlap trimming.
Document that overlap matching cannot identify a returning speaker who was
absent from the overlap, and add the reader-facing boundary to AGENTS.md.

Test Plan:
- python3 -B -m unittest -v: all 46 tests passed, including swapped IDs,
  ambiguous evidence, continuity across three requests, and plain headers.
- git diff --cached --check: passed.
- Tests use synthetic speaker labels; live diarization accuracy was not
  verified because provider credentials are unavailable in this environment.

test_chunks.py

@@ -6,7 +6,7 @@ import tempfile
 import unittest
 import wave
 
-from chunks import CONTEXT, ENCODING_MARGIN, merge, plan, stitch
+from chunks import CONTEXT, ENCODING_MARGIN, merge, plan, speaker_mapping, stitch
 from subtitles import convert
 
 
@@ -73,6 +73,45 @@ class ChunkPlanTests(unittest.TestCase):
 
 
 class ChunkMergeTests(unittest.TestCase):
+    def test_speaker_mapping_uses_shared_words_even_when_provider_ids_swap(self):
+        left = [dict(word(text, start), speaker=speaker) for text, start, speaker in (
+            ("one", 58, 0), ("two", 58.5, 0), ("three", 60, 1), ("four", 60.5, 1),
+        )]
+        right = [dict(w, speaker=1 - w["speaker"]) for w in left]
+        self.assertEqual(speaker_mapping(left, right, 60), {1: 0, 0: 1})
+
+    def test_speaker_mapping_rejects_missing_sparse_and_conflicting_evidence(self):
+        left = [dict(word(text, start), speaker=0) for text, start in (
+            ("one", 58), ("two", 58.5), ("three", 60), ("four", 60.5),
+        )]
+        self.assertEqual(speaker_mapping(left, [dict(left[0], speaker=5)], 60), {})
+        self.assertEqual(speaker_mapping(left, [dict(word("elsewhere", 90), speaker=0)], 60), {})
+        conflicting = [dict(w, speaker=i // 2) for i, w in enumerate(left)]
+        self.assertEqual(speaker_mapping(left, conflicting, 60), {})
+        self.assertEqual(speaker_mapping(conflicting, left, 60), {})
+        repeated = [dict(word("yes", 60), speaker=0)]
+        self.assertEqual(speaker_mapping(repeated, repeated * 2, 60), {})
+
+    def test_overlap_keeps_speaker_identity_through_three_requests(self):
+        chunks = [{"start": 0, "end": 63, "seam": 0},
+                  {"start": 57, "end": 120, "seam": 60},
+                  {"start": 114, "end": 140, "seam": 117}]
+        responses = [
+            {"words": [dict(word(text, start), speaker=7) for text, start in
+                       (("First", 1), ("shared", 59), ("words", 60))]},
+            {"words": [dict(word(text, start), speaker=2) for text, start in
+                       (("shared", 2), ("words", 3), ("more", 59), ("speech", 60))]},
+            {"words": [dict(word(text, start), speaker=9) for text, start in
+                       (("more", 2), ("speech", 3), ("Last", 20))]},
+        ]
+        original = copy.deepcopy(responses)
+        with contextlib.redirect_stderr(io.StringIO()) as errors:
+            result = merge(chunks, responses)
+        self.assertEqual(result["text"], "First shared words more speech Last")
+        self.assertEqual({w["speaker"] for w in result["words"]}, {0})
+        self.assertEqual(errors.getvalue(), "")
+        self.assertEqual(responses, original)
+
     def test_boundary_word_survives_once_despite_timestamp_drift(self):
         left = [word("before", 8), word("boundary,", 9.8, 10.4), word("after", 10.5)]
         right = [word("Boundary", 10.0, 10.6), word("after", 10.6), word("later", 14)]