Add typed VRP response model (Tier 1 endpoint 1 of full POCO sweep)

- VrpResponse + 8 sub-types (VrpCore, VrpDirectional, VrpTermItem,
  VrpGexConditioned, VrpVannaConditioned, VrpRegime, VrpStrategyScores,
  VrpMacro). Field-level nullability defensive across all 5 SDK languages.
- Rich docstrings on every type and field — explains semantics + units +
  sign conventions, calls out the famous nested-trap silent-null patterns
  (response.z_score → response.vrp.z_score; response.put_vrp → response
  .directional.downside_vrp; response.net_gex → response.regime.net_gex,
  etc). Boosts LLM autocomplete accuracy and adds searchable surface area
  on PyPI/typedoc/javadoc/NuGet.
- C# rename: VrpRegime.vrp_regime mapped to property VrpRegimeLabel
  because C# disallows a property name matching its enclosing class.
  JSON wire name unchanged.

Live vs historical structural diffs (encoded into the types):
- macro.fed_funds: live includes; historical omits
- macro.hy_spread: live currently null; historical populated (the
  earlier comment claiming hy_spread is hard-coded to 3.5 on historical
  is incorrect; corrected here)
- vrp.z_score / percentile / regime.vrp_regime / strategy_scores /
  net_harvest_score: nullable on historical when warmup is insufficient
  (queries near 2018-04-16, dataset start)

Backwards-compatible: existing untyped methods continue to return raw
JsonElement / map[string]interface{} / dict / unknown unchanged.
Integration tests + final 0.4.0-rc.2 release cut follow once Tier 1 is
complete (MaxPain + StockSummary still pending in this tier).

Author: tdobrowolski1

src/flashalpha/__init__.py

@@ -16,6 +16,15 @@
     ExposureSummaryInterpretation,
     ExposureSummaryResponse,
     ExposureSummaryZeroDte,
+    VrpCore,
+    VrpDirectional,
+    VrpGexConditioned,
+    VrpMacro,
+    VrpRegime,
+    VrpResponse,
+    VrpStrategyScores,
+    VrpTermItem,
+    VrpVannaConditioned,
     ZeroDteDecay,
     ZeroDteExpectedMove,
     ZeroDteExposures,
@@ -64,4 +73,14 @@
     "ExposureSummaryHedgingEstimate",
     "ExposureSummaryHedgingMove",
     "ExposureSummaryZeroDte",
+    # ── VRP ──
+    "VrpResponse",
+    "VrpCore",
+    "VrpDirectional",
+    "VrpTermItem",
+    "VrpGexConditioned",
+    "VrpVannaConditioned",
+    "VrpRegime",
+    "VrpStrategyScores",
+    "VrpMacro",
 ]