correlator: address drb-correlation-review notes on #139

- Document call_severity's routine-coercion asymmetry with incident_type
  (extraction-said-routine vs extraction-said-nothing look identical).
- Test docstring no longer overclaims the ctx-linkage it doesn't cover;
  points to the tests that do (test_consensus_gate.py, test_incident_identity.py).
- scene1 now uses a distinct severity so the test actually exercises both
  fields symmetrically; the 'no flat top-level clobber' claim is now
  asserted, not just commented.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tbknwttzou4s46PAykmtix

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

@@ -1467,6 +1467,13 @@ async def _apply_and_log(decision: dict, ctx: dict) -> Optional[str]:
                 # which is last-scene-wins (server-26#96) and was the reason
                 # #138's "type" veto couldn't be told apart from cross-scene
                 # contamination without re-guessing from a live dump.
+                # NOTE: unlike incident_type, call_severity is already
+                # coerced to "routine" when extraction emitted nothing
+                # (ctx build: `severity or "routine"`) — a scene reading
+                # "routine" here doesn't distinguish "extraction said
+                # routine" from "extraction said nothing". Don't split a
+                # severity veto the way #138 splits the type veto without
+                # accounting for that.
                 "incident_type": ctx.get("incident_type"),
                 "severity":      ctx.get("call_severity"),
             }