strategies: spending input is honoured + new "Custom" preset with guardrails

The user noticed the "Annual spending" field was a no-op for Trinity,
GK, VPW, VPW+floor — the strategies internally hardcoded the year-0
withdrawal as `initial_portfolio × initial_rate` (4% / 5.5%) and
ignored what the user typed. Two fixes:

(1) Trinity + GK now use state.initial_withdrawal (= the user's
    spending_target) as the year-0 draw. GK's guardrail anchor
    becomes the implied initial rate (initial_withdrawal /
    initial_portfolio), so the rule shape adapts to the user's
    chosen rate. Both strategies still fall back to their preset
    rate × initial_portfolio when initial_withdrawal isn't set
    (test paths). VPW and VPW+floor stay algorithmic — they're
    "withdraw-what's-sustainable" by design and don't take a
    spending input.

(2) New "custom" preset (SpendingPlanStrategy) exposing all the
    knobs:
    - initial_spend = "Annual spending" input
    - annual_real_adjust_pct = scale last year's withdrawal by N%
      each year (0 = constant real £, +0.02 = 2%/yr healthcare
      creep, -0.005 = -0.5%/yr slow-down with age)
    - guardrail_threshold_pct = if portfolio falls below X% of
      starting NW, trigger a cut (None = disabled)
    - guardrail_cut_pct = cut last year's withdrawal by Y% each
      triggered year

Adjust applies first, then guardrail cut — so a triggered year in
+2% adjust mode goes 40k → 40.8k → 36.7k.

UI: "custom" added to the strategy dropdown; when selected, three
extra fields appear (annual real adjustment %, guardrail trigger
threshold, guardrail cut size) with hints. The existing inputs
(spending, NW seed) drive year 0 across all strategies that use
them. About-the-model panel updated.

10 new tests on SpendingPlanStrategy + adjusted GK tests for the
new spending_target-aware behaviour. 209 backend tests + 7
frontend tests. mypy + ruff + tsc all pass.

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

fire_planner/scenarios.py

@@ -22,6 +22,7 @@ from fire_planner.glide_path import GLIDE_PATHS
 from fire_planner.simulator import RegimeFn, constant_regime, jurisdiction_schedule
 from fire_planner.strategies.base import WithdrawalStrategy
 from fire_planner.strategies.guyton_klinger import GuytonKlingerStrategy
+from fire_planner.strategies.spending_plan import SpendingPlanStrategy
 from fire_planner.strategies.trinity import TrinityStrategy
 from fire_planner.strategies.vpw import VpwStrategy, VpwWithFloorStrategy
 from fire_planner.tax.base import TaxRegime
@@ -58,7 +59,13 @@ class ScenarioSpec:
                 f"glide-{self.glide_path}")
 
 
-def build_strategy(name: str, floor: float | None = None) -> WithdrawalStrategy:
+def build_strategy(
+    name: str,
+    floor: float | None = None,
+    annual_real_adjust_pct: float = 0.0,
+    guardrail_threshold_pct: float | None = None,
+    guardrail_cut_pct: float = 0.10,
+) -> WithdrawalStrategy:
     if name == "trinity":
         return TrinityStrategy()
     if name == "guyton_klinger":
@@ -69,6 +76,12 @@ def build_strategy(name: str, floor: float | None = None) -> WithdrawalStrategy:
         if floor is None:
             raise ValueError("vpw_floor strategy requires a `floor` value (real GBP)")
         return VpwWithFloorStrategy(floor=floor)
+    if name == "custom":
+        return SpendingPlanStrategy(
+            annual_real_adjust_pct=annual_real_adjust_pct,
+            guardrail_threshold_pct=guardrail_threshold_pct,
+            guardrail_cut_pct=guardrail_cut_pct,
+        )
     raise KeyError(f"Unknown strategy: {name!r}")