feat(python): add PackageData.from_rez(pkg) convenience

[doubleailes]

Summary

Follow-up to #78. The integration site for rez was eight lines of boilerplate per package:

```python
pyrer.PackageData(
name=fam.name,
version=str(pkg.version),
requires=[str(r) for r in (pkg.requires or [])],
variants=[[str(r) for r in v] for v in (pkg.variants or [])],
)
```

Same body every time. Ship it as a classmethod on `PackageData` so the call site collapses to one line:

```python
pyrer.PackageData.from_rez(pkg)
```

Implementation

`#[classmethod] from_rez(cls, pkg: &Bound<', PyAny>)` reads four attributes off `pkg` via PyO3 duck typing — `pyrer` still does not import `rez`, so any object exposing `name` / `version` / `requires` / `variants` works (tests use plain Python classes, no rez install needed).

Handles in one place:

• `version` → `str(version)` (rez's `Version` is not a `str`).
• Each `Requirement` in `requires` and inside each `variants` entry → `str(req)`.
• `requires is None` / `variants is None` → empty list (rez models "no requires" as `None`).
• Missing one of the four attributes → `AttributeError` from PyO3 surfaces naturally.

Two small private helpers (`read_requirement_list`, `read_variants_list`) share the iteration logic.

Tests

Six new cases in `tests/test_rich_api.py`:

• plain attribute access (string fields, list-of-string requires)
• `requires`/`variants` is `None` → empty
• requirement-object `str` is honoured
• version-object `str` is honoured
• missing attribute raises `AttributeError`
• end-to-end: `from_rez` → `solve` produces the same result as the four-field constructor

72 / 72 Python tests pass (7 + 58 + 14).

Docs

`README.md` and `rez-integration.md` integration recipes shrink from eight lines of conversion to a one-line `PackageData.from_rez(pkg)` call. The `pyrer`-doesn't-import-rez note stays so users on non-rez pipelines know the constructor is still available for them.

🤖 Generated with Claude Code

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