correlator: address #102 review — 0-based segment labels, never-empty slice

drb-correlation-review on the prior commit flagged two ways the per-scene
transcript could silently fall back to the whole-call text:

1. _build_transcript_block numbered transmissions "1." while the prompt says
   "0-based indices" — a model echoing the labels it saw returned 1-based
   indices, shifting every scene's slice by one. Labels are now "0." to match
   the documented contract (also fixes the same latent skew in
   _build_scene_embed_text / #80).
2. An empty join (bad / out-of-range / non-int indices) hit
   `transcript or call_doc.get(...)` in _build_context and fell back to the
   whole-call transcript — re-opening the leak exactly when indices are wrong.
   The slice now falls back to this call's own whole transcript *before*
   _build_context sees it, so it is never "". Non-int and negative indices
   are rejected rather than raising.

Slice logic extracted to `_scene_transcript_text` with a dedicated test file
(4 cases: subset, corrected-wins, no-indices fallback, bad-indices fallback).
Call-doc fallback kept (sweep / no-scene path) per the review. Also restored
the `-> ` spacing lost in the prior commit's kwarg edit.

Full c2-core suite green: 300 passed (sandboxed venv). Still DO NOT MERGE
until the measurement window closes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

drb-c2-core/app/internal/intelligence.py

@@ -337,18 +337,9 @@ async def extract_scenes(
         )
         embedding = await asyncio.to_thread(_sync_embed, scene_text)
 
-        # This scene's own words, unprefixed — corrected text if we have it,
-        # else the raw segments this scene owns, else (single-scene) the whole
-        # transcript. The correlator's LLM tier reads this per scene instead of
-        # the call doc's whole-call transcript (server-26#102).
-        if transcript_corrected:
-            scene_transcript = transcript_corrected
-        elif segments and segment_indices:
-            scene_transcript = " ".join(
-                segments[i]["text"] for i in segment_indices if i < len(segments)
-            )
-        else:
-            scene_transcript = transcript
+        scene_transcript = _scene_transcript_text(
+            transcript, segments, segment_indices, transcript_corrected
+        )
 
         processed.append({
             "tags":                 tags,
@@ -585,11 +576,49 @@ def _municipality_from_tg(tg_name: Optional[str]) -> Optional[str]:
 def _build_transcript_block(transcript: str, segments: Optional[list[dict]]) -> str:
     """Format transcript as numbered transmissions if segments are available."""
     if segments and len(segments) > 1:
-        lines = [f"{i+1}. [{s['start']}s] {s['text']}" for i, s in enumerate(segments)]
+        # 0-based labels, matching the prompt's "0-based indices into the
+        # numbered transmissions" — the model echoes these back as
+        # `segment_indices`, which _build_scene_embed_text and the per-scene
+        # `transcript` (server-26#102) then slice with directly.
+        lines = [f"{i}. [{s['start']}s] {s['text']}" for i, s in enumerate(segments)]
         return f"Transmissions ({len(segments)}):\n" + "\n".join(lines)
     return f"Transcript:\n{transcript}"
 
 
+def _scene_transcript_text(
+    transcript: str,
+    segments: Optional[list[dict]],
+    segment_indices: Optional[list[int]],
+    transcript_corrected: Optional[str],
+) -> str:
+    """
+    This scene's own words, unprefixed — the segments it owns, joined.
+
+    server-26#102: the correlator's LLM tier reads this per scene instead of
+    the call doc's whole-call transcript, so on a multi-scene call scene N is
+    no longer judged against scenes 1..N-1's text.
+
+    Never returns "". Anything that would leave the slice empty — no
+    `segment_indices` (a single-segment call is never numbered by
+    `_build_transcript_block`), or indices that are out of range / not ints —
+    falls back to the whole-call transcript, which for a single-scene call is
+    the same text and for a mis-sliced multi-scene call is at least this
+    call's own words. `_sync_extract`'s prompt documents 0-based indices and
+    `_build_transcript_block` numbers to match, so no base normalisation here.
+    """
+    if transcript_corrected:
+        return transcript_corrected
+    if segments and segment_indices:
+        joined = " ".join(
+            segments[i]["text"]
+            for i in segment_indices
+            if isinstance(i, int) and 0 <= i < len(segments)
+        )
+        if joined:
+            return joined
+    return transcript
+
+
 def _build_scene_embed_text(
     transcript: str,
     segments: Optional[list[dict]],