feat(038): Phase 5 — US4 immutable resolver

- T022-T025: ScenarioGraph.Resolver.Resolve — typed parameter/selector/manual/
  remove-step resolutions emit immutable revisions linked via parent_revision_hash;
  stale base revision rejected (409 semantics); unrelated step ids/order unchanged
- Selector hints recorded in step description for auditability
- 58 scenario tests pass; ruff clean; regions balanced

backend/tests/services/dashboard_testing/scenario/test_resolver.py

@@ -0,0 +1,66 @@
+# #region Test.Scenario.Resolver [C:3] [TYPE Module] [SEMANTICS testing,scenario,resolver,revision]
+# @defgroup Test.Scenario Resolver immutable-revision tests.
+# @LAYER Test
+# @RELATION BINDS_TO -> [ScenarioGraph.Resolver.Resolve]
+# @RATIONALE Resolution produces immutable revisions; unrelated structure must never change.
+# @REJECTED Testing only happy-path resolution — would hide stale-base and unrelated-change regressions.
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+import pytest
+
+from pydantic import ValidationError
+
+from src.services.dashboard_testing.scenario.models import DashboardTestScenario
+from src.services.dashboard_testing.scenario.resolver import ResolveChange, resolve_scenario
+
+_FIXTURES = Path(__file__).resolve().parents[3] / "fixtures" / "dashboard_scenarios"
+
+
+def _load() -> DashboardTestScenario:
+    return DashboardTestScenario.model_validate(json.loads((_FIXTURES / "scenario_valid.json").read_text(encoding="utf-8")))
+
+
+def test_parameter_resolution_creates_new_revision() -> None:
+    sc = _load()
+    resolved = resolve_scenario(sc, [ResolveChange(kind="parameter", target="test_date", value="2026-08-01")])
+    assert resolved.revision_hash != sc.revision_hash
+    assert resolved.parent_revision_hash == sc.revision_hash
+    param = next(p for p in resolved.parameters if p.name == "test_date")
+    assert param.value == "2026-08-01"
+    assert param.status == "resolved"
+
+
+def test_unrelated_steps_unchanged() -> None:
+    sc = _load()
+    before = [(s.id, s.automation_status) for s in sc.steps]
+    resolved = resolve_scenario(sc, [ResolveChange(kind="parameter", target="counterparty", value="Beta LLC")])
+    after = [(s.id, s.automation_status) for s in resolved.steps]
+    assert before == after
+
+
+def test_selector_hint_resolution() -> None:
+    sc = _load()
+    resolved = resolve_scenario(sc, [ResolveChange(kind="selector", target="phase-2-B01-apply_filters", value="#filter-input")])
+    assert resolved.revision_hash != sc.revision_hash
+    assert resolved.parent_revision_hash == sc.revision_hash
+
+
+def test_stale_base_revision_rejected() -> None:
+    sc = _load()
+    with pytest.raises(ValueError, match="stale"):
+        resolve_scenario(sc, [ResolveChange(kind="parameter", target="test_date", value="2026-08-01")], base_revision_hash="0" * 64)
+
+
+def test_invalid_parameter_type_rejected() -> None:
+    sc = _load()
+    with pytest.raises(ValidationError):
+        resolve_scenario(sc, [ResolveChange(kind="parameter", target="test_date", value=12345)])
+
+
+def test_unknown_target_rejected() -> None:
+    sc = _load()
+    with pytest.raises(ValueError, match=r"unknown"):
+        resolve_scenario(sc, [ResolveChange(kind="parameter", target="nonexistent_param", value="x")])