Core-LB-G11: Add weighted distribution invariant tests

[RicheyWorks]

Overall Classification: WARN
Campaign: Core LoadBalancer Engineering Campaign
Goal: Core-LB-G11
Scope: test-hardening plus narrow contract documentation for weighted distribution invariants

Summary:

• Adds focused weighted distribution invariant tests for proportional positive weights.
• Covers zero-weight allocation when positive weights exist and all-zero equal fallback behavior.
• Characterizes invalid negative weight rejection before distribution.
• Verifies repeated weighted facade calls remain deterministic and allocations stay non-negative.
• Updates the core load-balancer feature contract to point reviewers at the new focused evidence.

Tests added/updated:

• Added CoreLoadBalancerWeightedDistributionInvariantTest.
• Updated docs/agent/CORE_LOADBALANCER_FEATURE_CONTRACT.md evidence references.

Verification:

• mvn test "-Dtest=CoreLoadBalancerWeightedDistributionInvariantTest" passed: 5 tests, 0 failures, 0 errors, 0 skipped.
• mvn test "-Dtest=CoreLoadBalancerWeightedDistributionInvariantTest,LoadBalancerTest,CoreLoadBalancerCrossStrategyEdgeInvariantTest,CoreLoadBalancerDeterministicOrderingTieTest,CoreLoadBalancerLeastLoadedSemanticsTest,AgentCoreLoadBalancerFeatureContractDocumentationTest,AgentCoreLoadBalancerReviewerEvidenceMapDocumentationTest,AgentCoreLoadBalancerEvidenceConsolidationDocumentationTest" passed: 109 tests, 0 failures, 0 errors, 0 skipped.
• mvn -q test passed.
• mvn -q "-DskipTests" package passed.
• mvn -B package passed: 2,536 tests, 0 failures, 0 errors, 0 skipped.
• git diff --check passed.
• git diff --cached --check passed.
• git diff --check origin/main...HEAD passed.
• .\scripts\smoke\enterprise-lab-workflow.ps1 -Package passed.

Safety:

• No production code changed.
• No Maven config changed.
• No CI/workflow changed.
• No Dockerfile/Compose changes.
• No scripts/runtime resources/endpoints/secrets/external targets/deployment/automation changed.
• PR Add next-level architecture skeleton #334 was not touched.
• Core-LB-G12 was not started in this PR.

Not-proven boundaries:

• No production readiness/certification.
• No live-cloud or real-tenant validation.
• No benchmark/load/stress proof.
• No throughput/p95/p99 proof.
• No real-world latency improvement claim.
• No replay/export proof.
• No broader automation.