Prepare 3.2.0 release

[gijzelaerr]

Summary

• finalize the 3.2.0 changelog, version, lockfile, and package metadata
• merge current master, including serial S7-200 PPI support (feat(ppi): add S7-200 serial transport #824), server COTP handshake hardening (fix(server): harden COTP connection framing #893), and correct invalid-range read errors (fix(server): return read item errors for invalid ranges #896)
• document the complete current public API, including PPI, advanced PLC operations, async usage, rate limiting, discovery, the demo server, SZL parsing, and DB helpers
• remove obsolete in-repository S7CommPlus documentation and dependencies, retaining only a migration redirect to the standalone project
• avoid import-time output when optional CLI dependencies are absent

Validation

• pre-commit run --all-files
• .venv/bin/mypy snap7 s7 tests example
• .venv/bin/pytest — 1412 passed, 49 skipped
• .venv/bin/sphinx-build -E -N -b html doc doc/_build -W
• uv lock --check --offline
• uv build --offline
• isolated wheel smoke test for version 3.2.0, the s7/snap7 aliases, PPI exports, and S7CommPlus exclusion

Release blocker

Keep this draft until #892 is merged through required review. Serial PPI remains explicitly documented as experimental and should be validated against target hardware before production use.