fix: format text transcripts and show speaker markers

Chunked text output previously joined every word into one line and discarded
speaker labels. Add a shared plain-text formatter that wraps lines, breaks
paragraphs at speaker changes and pauses, and splits long turns while
preserving word order. Display provider speaker IDs with chunk scope where
needed, without implying that IDs identify the same person across requests.

Request structured output for short diarized recordings too, and warn when
speaker labels are missing or incomplete. Keep SRT formatting separate and
preserve existing line breaks for short requests without diarization.

Test Plan:
- python3 -B -m unittest -q: all 43 tests passed.
- bash -n transcribe and git diff --cached --check: passed.
- Reflowed the existing transcript locally and verified all 15,888 words
  were preserved; source audio and generated transcripts are not committed.
- A live excerpt check was blocked before upload by unavailable 1Password
  access; actual provider diarization remains unverified.

chunks.py

@@ -9,6 +9,7 @@ import sys
 import wave
 
 from subtitles import convert, normalized_words
+from transcript import render_text
 
 CONTEXT = 3.0  # Audio on each side of a seam: six seconds shared by two uploads.
 ENCODING_MARGIN = 0.1  # MP3 encoder delay and frame padding at 16 kHz.
@@ -156,7 +157,7 @@ def main():
                 (args.manifest.parent / f"response-{i}.json").read_text(encoding="utf-8")
             ) for i in range(len(chunks))]
             response = merge(chunks, responses)
-            sys.stdout.write(convert(response) if args.srt else response["text"] + "\n")
+            sys.stdout.write(convert(response) if args.srt else render_text(response))
     except (ValueError, OSError, wave.Error) as error:
         parser.exit(1, f"Audio chunking failed: {error}\n")