Make compiler and assistant proper packages

[s0undsystem]

Closes #13. Reported by @jim-t-trevenx in #6.

compiler/ had no __init__.py, so reaching it meant putting the directory on sys.path, which both assistant/validator.py and tests/test_compiler.py did. That works until it doesn't: with compiler/ on the path, compile.py is a top-level module named compile, shadowing the builtin for anything that imports it.

What changed

Adds compiler/__init__.py and pyproject.toml. The compiler is now importable as compiler.compile, so the name cannot collide with the builtin, and neither consumer manipulates sys.path to find it. The tests keep a single sys.path entry pointing at the repo root, which is ordinary for running tests without installing, and pip install -e . makes even that redundant.

Nothing that worked before stops working. compile.py tries the package-relative import first and falls back to flat imports when executed as a script, so python compiler/compile.py still runs for the four workflows, the README, and anyone with the habit. python -m compiler.compile and a dlpac-compile console script are the tidier ways in.

requirements.txt now contains -e . instead of restating dependencies. pyproject.toml becomes the single place a version is pinned, and every existing pip install -r requirements.txt (four workflows, README, CONTRIBUTING) installs the project without any of them needing to change. anthropic stays optional, behind a claude extra.

Why this is worth doing

Not correctness, since nothing was broken. Distribution. An installable package is how someone tries this without cloning the repo, which is why I labelled the issue enhancement rather than bug.

Verification

Check	Result
python compiler/compile.py (backwards compat)	5 policies
python -m compiler.compile	5 policies
dlpac-compile (console script)	5 policies
tests/test_compiler.py	13/13
tests/test_assistant.py	12/12
DLPAC_BRAIN=dry-run python -m assistant.eval	2/2 drafts
Package imports from a cwd outside the repo	OK
import compile resolves to this project	No longer possible
Builtin compile()	Intact

Test-merged against both open PRs (#14, #15) with no conflicts, and the combined tree passes 18/18 compiler and 12/12 assistant tests.