Fix ontology lazy import

[dbose]

Fix Windows/core-install crashes in erwin import, and verify in a clean Docker install

Summary

A fresh Windows pip install modelith-dbt (core only, no [ontology] extra) crashed mdl import erwin in two different ways that our uv dev workspace could never reproduce. This PR fixes both, and adds an isolated Docker harness that exercises the flows in the install configuration users actually have, so this class of "green in dev, broken on install" bug is caught before shipping.

The bugs

1. ModuleNotFoundError: No module named 'pyoxigraph' — mdl import erwin (and mdl init) into an empty folder scaffolds .mdl/lock.yaml via mdl_ontology.lock.Lock. Importing that submodule ran mdl_ontology/__init__.py, which eagerly imported the RDF-backed providers chain (pyoxigraph, the optional [ontology] extra). A core install doesn't have it, so scaffolding crashed — breaking the promise that the core CLI runs without the extra.

2. OSError: [Errno 22] Invalid argument … \<root>.yaml — an erwin export carries a domain literally named <root>. The shared model writer (mdl_reverse.writer.write_model, used by every importer) used object names verbatim as filenames. < and > are illegal on Windows, so open() crashed. macOS/Linux silently wrote a <root>.yaml file, which is why local tests never caught it.

Traceback (most recent call last)
  File "C:\Users\XXXX\AppData\Roaming\Python\Python313\site-packages\mdl_cli\main.py", line 2956, in import_erwin_cmd
    write_reversed(result.model, write_target)

  File "C:\Users\XXXX\AppData\Roaming\Python\Python313\site-packages\mdl_reverse\writer.py", line 96, in write_model
    dump(f"logical/domains/{dom.name}.yaml", dom)

  File "C:\Users\XXXX\AppData\Roaming\Python\Python313\site-packages\mdl_reverse\writer.py", line 81, in dump
    path.write_text(dump_str(data), encoding="utf-8")

  File "C:\Program Files\Python313\Lib\pathlib\_local.py", line 555, in write_text
    return PathBase.write_text(self, data, encoding, errors, newline)

  File "C:\Program Files\Python313\Lib\pathlib\_abc.py", line 651, in write_text
    with self.open(mode='w', encoding=encoding, errors=errors, newline=newline) as f:

  File "C:\Program Files\Python313\Lib\pathlib\_local.py", line 537, in open
    return io.open(self, mode, buffering, encoding, errors, newline)

OSError: [Errno 22] Invalid argument:
'c:\\Users\\XXXX\\projects\\modelith-test\\model\\logical\\domains\\<root>.yaml'

The fixes

• mdl_ontology/__init__.py now exports every public name lazily (PEP 562 __getattr__). Importing a pyoxigraph-free member (Lock) no longer pulls the backend; a backend-dependent name imports it only when accessed. _ontology() in the CLI force-resolves one backend-dependent export inside its guard, so a genuinely missing backend still fails with the modelith-dbt[ontology] install hint (exit 4), not a raw traceback.
• mdl_reverse.writer now slugs every filename to a filesystem-safe form (strips < > : " / \ | ? *, control chars, trailing dots/spaces; never empty), and disambiguates collisions the slugging can create (Order Line + Order/Line → order_line.yaml + order_line-2.yaml). The object name is preserved in the model (a domain stays <root>); only the on-disk filename is sanitised. No names are dropped.

Verification (the durable part)

New sandbox/clean-install/ Docker harness: builds the wheel, installs it core-only (no extras) in python:3.12-slim, and runs the erwin import flow where a real user runs it — asserting the CLI loads, import into an empty folder scaffolds without the ontology extra, the <root> domain writes to a safe filename, and the model validates. ./sandbox/clean-install/run.sh does it in one command and exits non-zero on failure (pre-ship / CI gate). Verified passing on the current wheel, with a negative control confirming the container genuinely lacks pyoxigraph.

Tests

• test_core_without_ontology.py: added init + import erwin cases under a blocked RDF backend (the scaffold path that crashed).
• test_writer_filenames.py: illegal-char names → safe filenames + collision disambiguation.
• Full suite: 919 passed, 1 skipped; ruff clean.

Notes

• CLI modelith-dbt 0.6.8 → 0.6.10 (0.6.9 pyoxigraph, 0.6.10 filenames). VS Code extension unchanged at 0.3.16 — both fixes are CLI-side and the extension just shells out to mdl.
• Windows box: install the built modelith_dbt-0.6.10-py3-none-any.whl.