fix(_xorq): don't let optional plain-ibis interop deactivate the xorq branch

[hussainsultan]

Problem

from xorq.common.utils.ibis_utils import from_ibis, map_ibis sat inside the all-or-nothing HAS_XORQ import block in _xorq.py. But xorq's ibis_utils imports the plain ibis-framework stack (ibis, packaging, pyarrow_hotfix) at module scope.

In a minimal environment (xorq + BSL only, no plain ibis), that import raises ImportError — which took the entire xorq branch down with it. The result:

• HAS_XORQ read False even though xorq was fully importable
• the plain-ibis fallback shims ran against xorq objects
• to_tagged died at serialization/__init__.py:114 with AttributeError: 'Table' object has no attribute 'replace'

This is a dependency-activation bug, not an API mismatch — xorq's own node_utils.replace_nodes accepts Expr | Node on both xorq 0.3.34 and 0.3.37, where the failure was observed.

Changes

1. Move from_ibis/map_ibis into their own guard, after HAS_XORQ = True is set. When the plain-ibis stack is absent, only those two interop symbols degrade to raising stubs — _MapIbisXorqStub is registry-preserving so _patch_xorq_sortkey_compat() can still register handlers, matching the existing no-xorq _MapIbisStub contract.
2. Harden the no-xorq fallback replace_nodes to normalize Expr → op via to_node, matching xorq's native signature, so it can't crash on an expression even if a future un-gated caller hands it one.

Net: src/boring_semantic_layer/_xorq.py, +46/-2. No other files touched.

Validation

• xorq==0.3.34 + BSL with no plain ibis installed → HAS_XORQ True, xorq's native replace_nodes used, to_tagged works over a deferred_read_csv source.
• Plain-ibis-only env unchanged; the fallback now accepts both op and expr.
• 159 xorq / serialization / tagged tests pass on this branch standing on main.

Note

This branch was originally stacked on fix/serialization-rce-and-roundtrip and has been rebased directly onto main — it is independent of that work (disjoint files) and can merge on its own.

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