Fix transcript chunker context overflow; full-coverage extraction defaults

chunk_text split only on "\n\n", but ASR transcripts have none (speaker turns are joined by a single "\n"), so whole 2-3h episodes (~250K chars) went to the extractor in one call and 400'd on context overflow. Fall through paragraph -> line -> sentence -> word -> hard char-slice so no chunk exceeds the cap regardless of punctuation; guard max_chars < 1.

Default extraction to recall-first full coverage (chunk_chars 12K, max_chunks 999) and expose both as run-extract --chunk-chars / --max-chunks.

ROADMAP.md

@@ -33,6 +33,10 @@ falsification hypotheses (H1–H6) are in `DESIGN_v2.md`.
 - **Episode-pipelining** in `transcribe_worker` — download/chunk the next episode while transcribing the
   current one, to close the inter-episode GPU idle gap (the per-chunk 2-in-flight path is already done).
 - **Corpus-management UI** — add to the corpus over time and see the full corpus selection.
+- **Expose pipeline tunables in the UI (with the UI topic).** Extraction chunk size + per-doc chunk cap,
+  audio chunk length, audio concurrency, etc. are currently hardcoded defaults (now also CLI flags on
+  `run-extract`: `--chunk-chars`, `--max-chunks`). Surface them in the UI so they're visible/adjustable,
+  not black-box assumptions we forget about. Tie to the corpus-management UI work.
 - **Forward live operation** — the only real test: scoring un-pre-selected signals as they arrive, with
   the dual-evaluation ledger as arbiter.