Polish Sema paper flow

[hwesterb]

Summary

• Tighten Sema paper sections around the type-system framing, methodology, related work, and limitations.
• Preserve key pedagogic metaphors and intuitions while trimming over-dense prose.
• Regenerate paper/sema.pdf through the repository paper build script.

Validation

• bash scripts/compile_paper.sh -> 37 pages, 0 undefined refs
• git diff --check
• uv run --extra mcp --with pytest python -m pytest -> 237 passed

Notes

• The local pre-commit pytest hook invokes system Python 3.9 and fails on PEP 604 type hints already present in the codebase; the suite passes under the project Python 3.12 environment.