Proposal: Implement pre-server using keywords module

[koubaa]

Steps
Modify CI workflow to capture generated deck files in .github/workflows - Add a step after running pre tests/examples to upload generated .k files as workflow artifacts (from test/testfiles/output and example output directories).

Create a dedicated reference-generation script in examples - A Python script that runs each pre example (starting with examples/Explicit/ball_plate.py) and saves the generated deck to a known artifact collection location.

Add workflow artifact upload step - Configure GitHub Actions to archive the generated .k files as downloadable artifacts after the job completes.

STOPPING POINT: Run CI and download artifacts - Push the workflow changes, trigger CI, wait for completion, then download the generated reference deck files.

Create keywords-based backend in src/ansys/dyna/core/pre - Implement a keywords backend where DynaSolution, DynaMech, etc. build keyword objects internally using Deck. Server methods become no-ops or local file operations. Add backend selection flag (env var or config).

Add pre module tests in tests - Tests for the pre module that validate deck output against reference .k files stored in tests/testfiles/standard. These are proper pre module tests that happen to drive and validate the keywords backend conversion.

Start with ball_plate.py

Backend flag mechanism: Environment variable PYDYNA_PRE_BACKEND=keywords|grpc