feat(server): add V3 HMAC framing

[gijzelaerr]

Summary

• verify HMAC-SHA256 on incoming V3 server requests and sign every V3 response
• validate V3 response HMACs in the sync client, including reassembled Explore frames
• carry V3 response IntegrityIds without exposing them to response parsers
• add end-to-end SessionKey tests covering connect, read, write, and Explore

The emulator accepts an explicit 24-byte-or-longer session_key. It cannot derive the client random key from material encrypted for a real Siemens PLC without the corresponding private key, so this gives tests and emulator users matching negotiated key material.

Dependency

This is stacked on #775 and currently targets its branch. After #775 merges, this PR can be retargeted to master; any required update will merge master into this branch rather than rebase it.

Validation

• uv run --frozen pre-commit run --all-files
• uv run --frozen pytest -q (1764 passed, 82 skipped)
• uv build

Fixes #803