Python-side integration: add `rer_solver` fallback to rez `Resolver`

[doubleailes]

Problem

Once rer_solver Python module exists (#5), rez needs to actually call it.

Proposed Solution

Modify rez's src/rez/resolver.py:

try:
    import rer_solver
    _RUST_AVAILABLE = True
except ImportError:
    _RUST_AVAILABLE = False
 
class Resolver:
    def _solve_rust(self):
        result = rer_solver.solve(
            [str(r) for r in self.package_requests],
            [str(p) for p in self.package_paths],
            filters=self._serialize_filters(),
            max_iterations=10000,
        )
        if result.status == "solved":
            return self._map_rust_result(result)
        elif result.status == "failed":
            # Fall back to Python solver for better error reporting
            return self._solve_python()
        else:
            raise ResolveError(result.failure_description)
 
    def _map_rust_result(self, result):
        """Map (name, version, variant_index) tuples back to rez PackageVariant objects."""
        ...
 
    def _serialize_filters(self):
        """Convert PackageFilterList to list of (type, arg) tuples for Rust."""
        ...

Gate behind config.use_rust_solver (default False) and REZ_USE_RUST_SOLVER=1 env var.

Acceptance Criteria

• rez-env works with REZ_USE_RUST_SOLVER=1 for basic resolves
• Falls back to Python solver when Rust module isn't installed
• Falls back to Python solver on Rust solve failure (for better error messages)
• rez-env -v shows whether Rust solver was used
• Add use_rust_solver to rezconfig.py with default False