PR 4d: Finalize permanent deferral decisions for DEBT-003~006 and DEBT-008

Governance: Updated TECH_DEBT_REGISTER.md per AGENTS.md v16.0 cost-benefit analysis.

Deferred Items (4 items, 4pts):
- DEBT-003 (CA1305): Locale formatting via Serilog. Breaking change risk exceeds benefit.
- DEBT-004 (CA1707): xUnit test naming convention (underscores). Not a defect, acceptable standard.
- DEBT-005 (CA1861): Static readonly array allocations. Negligible perf; readability priority.
- DEBT-006 (xUnit2031): Assert.Single overload nit. Style preference, not safety-critical.

Accepted (1 item, 2pts):
- DEBT-008 (Namespace consistency): Per-project AssemblyName intentional for DLL clarity. Trade-off accepted.

Registry Status Summary (post-PR 4):
- Completed: 1 (1pt) — DEBT-001 CA1822 static methods
- No Action: 1 (1pt) — DEBT-002 CA1873 already compliant
- Deferred: 4 (4pts) — DEBT-003/004/005/006 (revisit if conditions change)
- Accepted: 1 (2pt) — DEBT-008 (no action needed)
- Backlog: 0 (0pts)

Q3 2026 Paydown: 1pt resolved (DEBT-001). Target 4pts (20% of 20pt total).
Strategy: Small additional quick-wins from deferred items could reach 4pt target if needed.

All 40 tests PASS (17 ModelOps + 18 SignalEngine + 5 Architecture)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Directory.Build.props

@@ -6,7 +6,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <AnalysisLevel>latest-recommended</AnalysisLevel>
-    <NoWarn>$(NoWarn);CA1873;CA1305;CA1707;CA1861;xUnit2031</NoWarn>
+    <NoWarn>$(NoWarn);CA1305;CA1707;CA1861;xUnit2031</NoWarn>
     <Deterministic>true</Deterministic>
     <ContinuousIntegrationBuild Condition="'$(CI)' == 'true'">true</ContinuousIntegrationBuild>
   </PropertyGroup>