test: establish domain fixture and pipeline test conventions

packages/decision-pipeline/src/fixtures.md

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+# Domain Fixture Conventions
+
+## Purpose
+
+This document establishes conventions for test fixtures used throughout Auto-Guardable.
+
+Fixtures provide predictable representations of domain inputs while keeping tests independent from production integrations.
+
+## Principles
+
+- Fixtures represent domain concepts, not implementation details.
+- Tests should describe behavior through contracts.
+- External systems should be replaced with deterministic test data.
+- Fixtures should remain small and intentional.
+
+## Current Pipeline Validation Flow
+
+```
+Domain Fixture
+      |
+      v
+Policy Result Fixture
+      |
+      v
+Decision Adapter
+      |
+      v
+Decision Pipeline
+      |
+      v
+Decision Output
+```
+
+## Future Usage
+
+Fixtures may later represent:
+
+- Camera observations
+- Sensor events
+- Policy scenarios
+- Action outcomes
+
+without requiring those systems to exist during foundation development.

packages/decision-pipeline/src/test-conventions.md

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+# Decision Pipeline Test Conventions
+
+## Purpose
+
+Define how future tests should validate pipeline behavior while preserving architectural boundaries.
+
+## Test Layers
+
+### Contract Tests
+
+Validate that interfaces remain stable.
+
+### Pipeline Tests
+
+Validate that information can move through the pipeline.
+
+### Integration Tests
+
+Validate collaboration between implemented components.
+
+## Rules
+
+- Tests should not depend on hardware.
+- Tests should not require external services.
+- Tests should prove behavior, not implementation details.
+- Each architectural boundary should have explicit validation.
+
+## Goal
+
+Future contributors should be able to add capabilities while maintaining confidence in the system's foundation.