Expose sessions through ILocalPeer

[flcl42]

Fixes #215.

Summary

• expose a stable live read-only session view through ILocalPeer
• synchronize Count and point-in-time enumeration with session lifecycle changes
• document the session-view semantics and cover mutation, concurrency, and lifecycle behavior

Validation

• built all 45 non-MAUI Release/CI projects successfully
• passed Libp2p.Core.Tests: 42/42
• passed Libp2p.E2eTests: 6/6
• passed independent concurrent-access and two-sided lifecycle probes

CI status

The Test workflow currently stops during restore on the unchanged main dependency baseline because SIPSorcery 10.0.3 raises two security advisories under warnings-as-errors. #208 contains the dependency/API update and has green Format and Test checks. After #208 lands, this branch can be rebased onto main and its checks rerun without duplicating that unrelated change here.

[flcl42]

@copilot

[flcl42]

Closing in favor of a fresh issue #215 workflow run that incorporates the complete review and self-improvement cycle.