test(voyage): add 4 brief fixtures for v5.1.1 runtime scenarios

plugins/voyage/tests/fixtures/brief-v21-no-signals.md

@@ -0,0 +1,32 @@
+---
+type: trekbrief
+brief_version: "2.1"
+created: 2026-05-14
+task: "Fixture: v5.1 brief WITHOUT phase_signals or partial (falsification target)"
+slug: brief-v21-no-signals
+project_dir: .claude/projects/2026-05-14-brief-v21-no-signals/
+research_topics: 0
+research_status: complete
+auto_research: false
+interview_turns: 4
+source: fixture
+---
+
+# Task: brief_version 2.1 without phase_signals
+
+## Intent
+
+Falsification fixture for the v5.1 sequencing gate. The brief declares
+`brief_version: "2.1"` but omits BOTH `phase_signals` AND
+`phase_signals_partial: true`. The brief-validator MUST emit
+`BRIEF_V51_MISSING_SIGNALS` for this file — the runtime test for the
+sequencing gate asserts the error code fires.
+
+## Goal
+
+Validate that brief-validator catches the missing-signals scenario.
+
+## Success Criteria
+
+- brief-validator returns valid: false.
+- errors contains BRIEF_V51_MISSING_SIGNALS.