refactor: rename precompile surface to preprocess

[rlippmann]

What changed

• Renamed remaining experimental preprocessor API vocabulary from precompile to preprocess across code, tests, fixtures, docs, and integration examples.
• Removed legacy compatibility surface:
  • removed experimental/preprocessor/heuristic_precompiler.py
  • removed legacy precompiler naming references from current code/tests/contracts/docs
• Updated canonical names and exports to preprocessor-only terminology.
• Bumped package version to 0.6.19 in pyproject.toml and uv.lock.

Why

• Align terminology before TS preprocessor porting to avoid carrying transitional naming into first non-Python implementation.
• Reduce ambiguity by establishing a single canonical naming surface (preprocessor/preprocess) for 0.6.19.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)