M1: pure normalization engine + default seed

Infrastructure-free normalization engine in Core dispatching on MeterMode:
- Cumulative/Generation counters: register deltas, first-reading baseline (Haus 411,
  Auto 3755), meter swaps (water …861→2 reconciles to 12 via boundary registers OR an
  explicit amount override), counter resets, anomaly-guarded decreases.
- RuntimeCounter: Δhours × rate (fixed/empirical).
- ConsumableBalance: tank level-Δ + deliveries → consumption, cm→litre calibration;
  delivery-only rows before the first dipstick emit nothing.
- DirectDelta, and Virtual meters via a small safe arithmetic evaluator (Netz Einsparung
  = Haus − Netz, Eigenverbrauch = Erzeugung − Einsparung) — data-driven, not hardcoded.
- DatabaseSeeder: default energy types, cost categories, base settings (idempotent).

24 Core unit tests + 4 integration tests green.

Claude-Session: https://claude.ai/code/session_01WujdMtMJPbxDpDnMeK22rr

tests/Core.Tests/WaterSwapTests.cs

@@ -0,0 +1,57 @@
+using MeterVault.Core.Domain;
+using MeterVault.Core.Normalization;
+using static MeterVault.Core.Tests.TestData;
+
+namespace MeterVault.Core.Tests;
+
+/// <summary>
+/// The water meter register swaps mid-series (…861 → 2 → 15). The swap month's consumption (12)
+/// cannot be derived from the two visible registers alone — an explicit swap event carries the
+/// boundary, and continuity is preserved across it (SDD §2.3, §7.1).
+/// </summary>
+public sealed class WaterSwapTests
+{
+    private readonly INormalizationEngine _engine = NormalizationEngine.CreateDefault();
+
+    private static NormalizationContext WaterContext(MeterEvent swap) => new()
+    {
+        Meter = new MeterConfig
+        {
+            MeterId = 10,
+            Mode = MeterMode.CumulativeCounter,
+            Unit = "m3",
+            InitialBaseline = 820, // Nov 2022 register, so Dez books 14.
+        },
+        Readings =
+        [
+            Reading(10, Month(2022, 12), 834),
+            Reading(10, Month(2023, 1), 848),
+            Reading(10, Month(2023, 2), 861),
+            Reading(10, Month(2023, 3), 2),   // new meter
+            Reading(10, Month(2023, 4), 15),
+        ],
+        Events = [swap],
+    };
+
+    [Fact]
+    public void Swap_with_boundary_registers_reconciles_to_twelve()
+    {
+        // Old meter ran 861 → 873 before removal; new meter installed reading 2.
+        var swap = Swap(10, Month(2023, 3), prevValue: 873, newValue: 2);
+
+        var result = _engine.Normalize(WaterContext(swap));
+
+        Assert.Equal([14d, 14d, 13d, 12d, 13d], result.Select(c => c.Amount));
+    }
+
+    [Fact]
+    public void Swap_with_explicit_amount_override_reconciles_to_twelve()
+    {
+        // The importer can seed the sheet's own März consumption (12) as an override.
+        var swap = Swap(10, Month(2023, 3), amount: 12);
+
+        var result = _engine.Normalize(WaterContext(swap));
+
+        Assert.Equal([14d, 14d, 13d, 12d, 13d], result.Select(c => c.Amount));
+    }
+}