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/tests/test_scene_transcript.py

@@ -0,0 +1,40 @@
+"""
+server-26#102 — a scene is correlated on its OWN transcript, not the whole call.
+
+_scene_transcript_text slices the segments a scene owns. It must never return
+"" (an empty slice would let incident_correlator._build_context fall back to
+the call doc's whole-call transcript, re-opening the leak in exactly the case
+— bad indices — where it matters).
+"""
+from app.internal.intelligence import _scene_transcript_text
+
+SEGS = [
+    {"text": "structure fire, 12 Main"},
+    {"text": "engine 4 responding"},
+    {"text": "traffic stop, plate ABC"},
+    {"text": "one occupant"},
+]
+WHOLE = "structure fire, 12 Main engine 4 responding traffic stop, plate ABC one occupant"
+
+
+def test_scene_owns_a_subset_of_segments():
+    assert _scene_transcript_text(WHOLE, SEGS, [0, 1], None) == "structure fire, 12 Main engine 4 responding"
+    assert _scene_transcript_text(WHOLE, SEGS, [2, 3], None) == "traffic stop, plate ABC one occupant"
+
+
+def test_corrected_text_wins_when_present():
+    assert _scene_transcript_text(WHOLE, SEGS, [0], "cleaned up text") == "cleaned up text"
+
+
+def test_no_segment_indices_falls_back_to_whole_call():
+    # single-segment calls are never numbered by _build_transcript_block → null indices
+    assert _scene_transcript_text(WHOLE, SEGS, None, None) == WHOLE
+    assert _scene_transcript_text(WHOLE, None, [0, 1], None) == WHOLE
+
+
+def test_out_of_range_or_nonint_indices_fall_back_never_empty():
+    assert _scene_transcript_text(WHOLE, SEGS, [9, 10], None) == WHOLE          # all out of range
+    assert _scene_transcript_text(WHOLE, SEGS, ["1", "2"], None) == WHOLE       # 1-based strings, rejected
+    assert _scene_transcript_text(WHOLE, SEGS, [-1], None) == WHOLE            # negative
+    # partial validity: keep what's in range
+    assert _scene_transcript_text(WHOLE, SEGS, [3, 99], None) == "one occupant"