feat(phase2): llm report writer (P5.3)

- New module src/pf1e_simulator/report.py with ReportContext, build_report_context,
  LLMReportPolicy, ReportBudget, format_llm_report, build_report_prompt
- Prompt pack fixture: tests/fixtures/prompt_pack_fs/report.md (FR narrator)
- CLI integration: --llm-report + --output flags, mutually exclusive with --transcript
- Budget enforcement: ReportBudget integrates with CostBudget, allows max_retries+1 calls
- Tests: test_report_context, test_report_prompt_pack, test_llm_report_policy,
  test_format_llm_report, test_cli_llm_report, test_report_budget (27 new tests)

tests/test_format_llm_report.py

@@ -0,0 +1,63 @@
+"""Tests for format_llm_report (P5.3 T4)."""
+
+from __future__ import annotations
+
+from pf1e_simulator.report import build_report_context, format_llm_report
+from pf1e_simulator.runner import CombatantAttrition, EncounterReport
+
+
+def _make_encounter() -> EncounterReport:
+    return EncounterReport(
+        runs=100,
+        seed=1,
+        wins={"players": 65, "monsters": 33},
+        draws=2,
+        avg_rounds=7.2,
+        attrition={
+            "goblin": CombatantAttrition(2.45, 0.12, 18.30, 4.20, 0.00),
+            "orc": CombatantAttrition(3.20, 0.25, 28.40, 15.60, 0.00),
+        },
+    )
+
+
+def test_format_llm_report_empty_narrative() -> None:
+    """Empty LLM text returns only the factual report."""
+    ctx = build_report_context(encounter=_make_encounter(), spec_name="test")
+    result = format_llm_report(ctx, "")
+
+    assert "=== Rapport d'équilibrage ===" in result
+    assert "Victoires players : 65.0%" in result
+    assert "Récit d'équilibrage" not in result
+
+
+def test_format_llm_report_whitespace_only_narrative() -> None:
+    """Whitespace-only LLM text returns only the factual report."""
+    ctx = build_report_context(encounter=_make_encounter(), spec_name="test")
+    result = format_llm_report(ctx, "   \n  \t  ")
+
+    assert "=== Rapport d'équilibrage ===" in result
+    assert "Récit d'équilibrage" not in result
+
+
+def test_format_llm_report_with_narrative() -> None:
+    """Non-empty LLM text appends narrative section after factual report."""
+    ctx = build_report_context(encounter=_make_encounter(), spec_name="test")
+    narrative = "Le gobelin a surpris l'orc par sa vitesse."
+    result = format_llm_report(ctx, narrative)
+
+    assert "=== Rapport d'équilibrage ===" in result
+    assert "Victoires players : 65.0%" in result
+    assert "=== Récit d'équilibrage (LLM) ===" in result
+    assert narrative in result
+    # factual comes before narrative
+    factual_end = result.index("=== Récit d'équilibrage (LLM) ===")
+    assert "Victoires players" in result[:factual_end]
+
+
+def test_format_llm_report_strips_narrative_whitespace() -> None:
+    """Leading/trailing whitespace in narrative is stripped."""
+    ctx = build_report_context(encounter=_make_encounter(), spec_name="test")
+    result = format_llm_report(ctx, "  \n  Texte narratif.  \n  ")
+
+    assert result.endswith("Texte narratif.")
+    assert "=== Récit d'équilibrage (LLM) ===\nTexte narratif." in result