Split S7CommPlus into standalone s7commplus package, remove unified s7 client

[gijzelaerr]

Summary

The current s7/ package tries to unify legacy S7 (snap7) and S7CommPlus behind a single Client that auto-negotiates the protocol. In practice the two protocols differ too much — different capabilities (browse is S7CommPlus-only), different connection requirements (TLS, SessionKey handshake), and different failure modes. The silent fallback from S7CommPlus to legacy creates confusing errors (#760, #710) and makes it hard for users to understand what's happening.

Plan

1. Create a new top-level s7commplus/ package containing the S7CommPlus-specific code currently in s7/:

   • _s7commplus_client.py → s7commplus/client.py
   • _s7commplus_async_client.py → s7commplus/async_client.py
   • _s7commplus_server.py → s7commplus/server.py
   • s7/connection.py → s7commplus/connection.py
   • s7/protocol.py → s7commplus/protocol.py
   • s7/codec.py, s7/vlq.py, s7/legitimation.py → s7commplus/

2. Keep snap7/ unchanged — it remains the legacy S7 package for S7-300/400 and PUT/GET on S7-1200/1500.

3. Shared code (TPKT/COTP transport in snap7/connection.py) stays in snap7 — s7commplus imports from it.

4. Remove the unified s7/ package — no more s7/client.py with auto-negotiation and silent fallback. Users explicitly choose snap7.Client or s7commplus.Client.

5. Update examples, docs, tests, and CLAUDE.md to reflect the new structure.

Motivation

• Users should explicitly choose their protocol rather than getting silent fallback behavior
• The two protocols have fundamentally different capabilities and requirements
• Cleaner API surface — no more "browse() requires S7CommPlus connection" errors after silent fallback
• Easier to document and maintain each protocol independently

Timing

Do this after all current open PRs are merged (#761, #763, #764, #768) since they touch s7/client.py and s7/connection.py heavily.

Breaking change

This removes the s7 package entirely. Users importing from s7 will need to switch to either snap7 or s7commplus. This should be part of a major or minor version bump with migration notes.