Add controlled Whisper and LLM benchmark results

Document and preserve the controlled Progeo benchmark series.

Whisper comparison:
- Compare whisper.cpp large-v3 and large-v3-turbo
- Identify and verify BUG-012: extraction stability depends on chunk size
- Repeat both transcription variants with identical reduced chunk budgets
- Select large-v3-turbo as the current production transcription model

LLM comparison:
- Compare qwen3.5:9b with qwen3.5:35B-A3B
- Preserve identical transcript, Meeting Context, prompts and chunking
- Retain qwen3.5:9b as the production recommendation
- Record runtime, repair burden and semantic-quality findings

Current production benchmark configuration:
- whisper.cpp large-v3-turbo
- target_chars=4500
- max_chars=5500
- min_chars=2500
- overlap_blocks=0
- qwen3.5:9b
- num_ctx=32768
- think=false
- temperature=0

BUG-012 remains verified but not yet fixed in the production chunker.

docs/regression-bugs.md

@@ -839,3 +839,95 @@ Current status:
 Verified. The renderer contract is now enforced deterministically. This does
 not improve semantic quality of the generated prose; it prevents invalid
 renderer output from being accepted as a final Working Protocol.
+
+## BUG-012
+
+ID: BUG-012
+
+Title: Chunking sensitivity to Whisper segmentation density
+
+Pipeline stage: Chunking / Extraction
+
+Severity: High
+
+Status: Verified hypothesis
+
+Date discovered: 2026-08-05
+
+Version first observed: `progeo_whisper_turbo_20260805_120903`
+
+Description:
+
+Experiment 1 compared two valid trimmed whisper.cpp transcriptions while
+keeping Meeting Context, source code, prompts, LLM, model parameters, pipeline
+stages, validation/repair behavior and renderer configuration identical.
+
+The large-v3 transcription produced 3,822 input segments, 216,500 segment-text
+characters and 25 chunks. It completed extraction, canonicalization, Semantic
+Consolidator V0 and renderer invocation under the identical downstream
+settings.
+
+The large-v3-turbo transcription produced 1,211 input segments, 83,473
+segment-text characters and 10 chunks. Extraction failed at `chunk_04`. The
+preserved raw response repeated the same fact many times and truncated
+mid-string, producing invalid JSON. Because the extraction output for
+`chunk_04` was invalid, canonicalization, semantic consolidation and rendering
+could not proceed.
+
+Expected behaviour:
+
+Chunk construction should provide stable extraction inputs across reasonable
+transcription segment-density differences. A valid, shorter Turbo transcript
+should not fail extraction solely because its segment boundaries create
+different chunk shapes.
+
+Actual behaviour:
+
+The pipeline was materially more compatible with the large-v3 transcript than
+with the Turbo transcript. The Turbo run failed before downstream stages even
+though the downstream settings were unchanged.
+
+Important interpretation:
+
+This does not prove that whisper.cpp large-v3 is intrinsically better than
+large-v3-turbo. It proves only that large-v3 was more compatible with the
+current chunking/extraction path in this controlled run.
+
+Suspected root cause:
+
+The current chunker treats Whisper JSON segments as atomic blocks and builds
+chunks by character budget around those source segments. Different Whisper
+segment density therefore changes the effective extraction input shape. The
+suspected failure mode is chunk/token budgeting and extraction stability, not
+merely transcription readability or Whisper quality.
+
+Related files:
+
+- `samples/real_live/progeo_meeting/progeo_whispercpp_vulkan_large_v3_trimmed_converted.json`
+- `samples/real_live/progeo_meeting/progeo_whispercpp_vulkan_turbo_trimmed_converted.json`
+- `samples/benchmarks/progeo_whisper_large_v3_20260805_120903/`
+- `samples/benchmarks/progeo_whisper_turbo_20260805_120903/`
+- `samples/benchmarks/progeo_whisper_turbo_small_chunks_20260805_124324/`
+- `samples/benchmarks/progeo_whisper_turbo_20260805_120903/extractions/chunk_04_extraction.raw.txt`
+- `samples/benchmarks/progeo_whisper_comparison_20260805_120903_metrics.json`
+- `src/meeting_lab/chunking/chunk_transcript.py`
+
+Regression test available (yes/no): no
+
+Current status:
+
+Verified hypothesis. Experiment 1B reran the same Turbo transcription through
+the same downstream settings while changing only the chunk budget from the
+default `target_chars=9000`, `max_chars=11000`, `min_chars=5000`,
+`overlap_blocks=0` to `target_chars=4500`, `max_chars=5500`,
+`min_chars=2500`, `overlap_blocks=0`.
+
+The reduced-size Turbo run produced 19 chunks instead of 10. All extraction
+chunks produced valid JSON, canonicalization completed, Semantic Consolidator
+V0 completed with existing deterministic source-coverage repair, and the
+renderer was invoked. Renderer contract validation still failed because the
+candidate did not start with `# Working Protocol`; that remains covered by
+BUG-011 and does not invalidate this chunking hypothesis.
+
+This verifies the hypothesis for this transcript/configuration only. It does
+not claim the general chunker is fixed.