Phase 6.1 Stream B.1/B.2 — DriverTier on DriverTypeMetadata + Core.Stability.MemoryTracking with hybrid-formula soft/hard thresholds

Stream B.1 — registry invariant:
- DriverTypeMetadata gains a required `DriverTier Tier` field. Every registered
  driver type must declare its stability tier so the downstream MemoryTracking,
  MemoryRecycle, and resilience-policy layers can resolve the right defaults.
  Stamped-at-registration-time enforcement makes the "every driver type has a
  non-null Tier" compliance check structurally impossible to fail.
- DriverTypeRegistry API unchanged; one new property on the record.

Stream B.2 — MemoryTracking (Core.Stability):
- Tier-agnostic tracker per decision #146: captures baseline as the median of
  samples collected during a post-init warmup window (default 5 min), then
  classifies each subsequent sample with the hybrid formula
  `soft = max(multiplier × baseline, baseline + floor)`, `hard = 2 × soft`.
- Per-tier constants wired: Tier A mult=3 floor=50 MB, Tier B mult=3 floor=100 MB,
  Tier C mult=2 floor=500 MB.
- Never kills. Hard-breach action returns HardBreach; the supervisor that acts
  on that signal (MemoryRecycle) is Tier C only per decisions #74, #145 and
  lands in the next B.3 commit on this branch.
- Two phases: WarmingUp (samples collected, Warming returned) and Steady
  (baseline captured, soft/hard checks active). Transition is automatic when
  the warmup window elapses.

Tests (15 new, all pass):
- Warming phase returns Warming until the window elapses.
- Window-elapsed captures median baseline + transitions to Steady.
- Per-tier constants match decision #146 table exactly.
- Soft threshold uses max() — small baseline → floor wins; large baseline →
  multiplier wins.
- Hard = 2 × soft.
- Sample below soft = None; at soft = SoftBreach; at/above hard = HardBreach.
- DriverTypeRegistry: theory asserts Tier round-trips for A/B/C.

Full solution dotnet test: 963 passing (baseline 906, +57 net for Phase 6.1
Stream A + Stream B.1/B.2). Pre-existing Client.CLI Subscribe flake unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/ZB.MOM.WW.OtOpcUa.Core.Abstractions.Tests/DriverTypeRegistryTests.cs

@@ -7,11 +7,13 @@ public sealed class DriverTypeRegistryTests
 {
     private static DriverTypeMetadata SampleMetadata(
         string typeName = "Modbus",
-        NamespaceKindCompatibility allowed = NamespaceKindCompatibility.Equipment) =>
+        NamespaceKindCompatibility allowed = NamespaceKindCompatibility.Equipment,
+        DriverTier tier = DriverTier.B) =>
         new(typeName, allowed,
             DriverConfigJsonSchema: "{\"type\": \"object\"}",
             DeviceConfigJsonSchema: "{\"type\": \"object\"}",
-            TagConfigJsonSchema: "{\"type\": \"object\"}");
+            TagConfigJsonSchema: "{\"type\": \"object\"}",
+            Tier: tier);
 
     [Fact]
     public void Register_ThenGet_RoundTrips()
@@ -24,6 +26,20 @@ public sealed class DriverTypeRegistryTests
         registry.Get("Modbus").ShouldBe(metadata);
     }
 
+    [Theory]
+    [InlineData(DriverTier.A)]
+    [InlineData(DriverTier.B)]
+    [InlineData(DriverTier.C)]
+    public void Register_Requires_NonNullTier(DriverTier tier)
+    {
+        var registry = new DriverTypeRegistry();
+        var metadata = SampleMetadata(typeName: $"Driver-{tier}", tier: tier);
+
+        registry.Register(metadata);
+
+        registry.Get(metadata.TypeName).Tier.ShouldBe(tier);
+    }
+
     [Fact]
     public void Get_IsCaseInsensitive()
     {