engine+api: plumb life events into the simulator

Until now life events were stored but ignored by the engine — pure
metadata. Now they actually move portfolios.

Engine:
- simulator.simulate() takes optional cashflow_adjustments: a (n_years,)
  real-GBP array applied each year *after* savings + return but
  *before* withdrawal. Positive = inflow, negative = outflow.
- New fire_planner/life_events.py with EventInput dataclass +
  events_to_cashflow_array(events, horizon). Handles ranged deltas,
  one-time amounts, disabled events, year clipping past horizon,
  negative year_start (clipped to 0), and summing multiple events.

API:
- /simulate accepts optional life_events list. Server converts each
  to EventInput, builds cashflow_adjustments, passes to simulate().
- Frontend Run-now on scenario detail now fetches the scenario's
  life events and includes them in the request — projections finally
  reflect "retire at 50, kid born at y3, inheritance at y22".

Tests: 11 events helper + 4 end-to-end engine + 1 API integration =
16 new tests. 188 total (was 172). mypy strict + ruff clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -229,6 +229,13 @@ export interface SimulateRequest {
   floor_gbp?: string | null;
   n_paths?: number;
   seed?: number;
+  life_events?: Array<{
+    year_start: number;
+    year_end?: number | null;
+    delta_gbp_per_year?: string;
+    one_time_amount_gbp?: string | null;
+    enabled?: boolean;
+  }>;
 }
 
 export interface SimulateResult {

frontend/src/pages/ScenarioDetail.tsx

@@ -7,7 +7,7 @@
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 import { Link, useNavigate, useParams } from 'react-router-dom';
 
-import { api, type Scenario, type SimulateRequest } from '@/api/client';
+import { api, lifeEventsApi, type Scenario, type SimulateRequest } from '@/api/client';
 import { ApiError } from '@/api/client';
 import { FanChart } from '@/components/FanChart';
 import { GoalsSection } from '@/components/GoalsSection';
@@ -53,7 +53,9 @@ export function ScenarioDetail() {
     del.mutate();
   };
 
-  const onRunNow = (s: Scenario) =>
+  const onRunNow = async (s: Scenario) => {
+    // Pull events fresh so the run reflects whatever the user just edited.
+    const events = await lifeEventsApi.list(s.id);
     sim.mutate({
       jurisdiction: s.jurisdiction,
       strategy: s.strategy,
@@ -65,7 +67,15 @@ export function ScenarioDetail() {
       horizon_years: s.horizon_years,
       n_paths: 5000,
       seed: 42,
+      life_events: events.map((e) => ({
+        year_start: e.year_start,
+        year_end: e.year_end,
+        delta_gbp_per_year: e.delta_gbp_per_year,
+        one_time_amount_gbp: e.one_time_amount_gbp,
+        enabled: e.enabled,
+      })),
     });
+  };
 
   if (!Number.isFinite(id)) {
     return <p className="text-red-700">Invalid scenario id.</p>;
@@ -104,9 +114,10 @@ export function ScenarioDetail() {
         <div className="flex items-center gap-2">
           <button
             type="button"
-            onClick={() => onRunNow(s)}
+            onClick={() => void onRunNow(s)}
             disabled={sim.isPending}
             className="rounded-md border border-slate-300 bg-white text-sm px-3 py-1.5 hover:bg-slate-50 disabled:opacity-60"
+            title="Run a fresh MC including this scenario's life events"
           >
             {sim.isPending ? 'Running…' : 'Run now'}
           </button>