Promote the DuckDB IEJoin INTERSECTS implementation to a registered (DuckDBTarget, Intersects) override expander

[conradbzura]

Description

Migrate the DuckDB IEJoin column-to-column INTERSECTS join rewrite (IntersectsDuckDBIEJoinTransformer) from a pre-pass transformer into a registered, target-specific (DuckDBTarget, Intersects) override expander in the operator-expander registry. This is the DuckDB counterpart to #167 (which makes the naive overlap predicate the GenericTarget default and removes binning); together they move all INTERSECTS join strategy out of the pre-pass transformers and into the registry.

End state:

• The DuckDB IEJoin rewrite is a registered (DuckDBTarget, Intersects) override that supersedes the GenericTarget naive-predicate default (Drop the binned INTERSECTS join path in favor of the naive overlap predicate #167) for the shapes it supports.
• For shapes it declines, it defers to the GenericTarget default (the naive predicate) rather than the removed binned transformer.
• Outer-join bail-out: the IEJoin rewrite cannot express outer-join INTERSECTS (_has_outer_join_intersects, transformer.py:82), so the override declines LEFT/RIGHT/FULL joins and defers to the naive-predicate default (which handles outer correctly per Drop the binned INTERSECTS join path in favor of the naive overlap predicate #167). This reframes Support LEFT/RIGHT JOIN in the DuckDB IEJoin dialect #95 ("Support LEFT/RIGHT JOIN in the DuckDB IEJoin dialect") as "defer to the default" and likely moots it.
• The global uses_iejoin strategy branch in transpile.py (~156, 214-235) is removed in favor of the registry override via ExpanderRegistry.has_override (transpile.py:181).

Motivation

Complete the operator-expander registry migration. The registry scaffolding (#138) and per-operator migrations landed for DISTANCE (#140), the INTERSECTS/CONTAINS/WITHIN predicates (#141), NEAREST (#142), DISJOIN (#143), and CLUSTER/MERGE (#144) — but the INTERSECTS join strategies (binned + DuckDB IEJoin) were left as pre-pass transformers selected by a hardcoded global capability branch. #167 removes the binned pre-pass and makes the GenericTarget default the naive predicate; this issue removes the IEJoin pre-pass and re-homes it as a registered DuckDB override, so INTERSECTS join strategy is fully registry-driven and target-overridable.

Expected Outcome

• DuckDB column-to-column INTERSECTS joins are produced by a registered (DuckDBTarget, Intersects) override expander, not the pre-pass transformer.
• The override declines outer joins (and any shape it cannot express) and defers to the GenericTarget naive-predicate default; no reliance on the removed binned transformer.
• The global uses_iejoin branch in transpile.py is removed; strategy selection is via has_override.
• Existing IEJoin results for supported shapes are preserved (regression), verified against DuckDB (oracle/integration test).

Interaction with existing IEJoin-dialect issues

With #167's naive predicate as the universal fallback, shapes the IEJoin override declines now defer to a capable default rather than the removed binned transformer. Several open IEJoin-dialect limitation issues may become closeable as "handled by the default" instead of requiring IEJoin support: #95 (LEFT/RIGHT JOIN), #96 (multiple INTERSECTS), #97 (self-joins), #98 (extra ON/WHERE predicates), #99 (bare */unqualified columns), #109 (top-level modifiers), #110 (CTEs/subquery operands), #111 (3+ tables). This issue makes a per-shape keep-in-IEJoin vs defer-to-default decision.

Relevant code

• src/giql/transformer.py — IntersectsDuckDBIEJoinTransformer, _IEJoinSides, _has_outer_join_intersects, transform_to_sql (decline/None fallback), _classify_extras
• src/giql/transpile.py — uses_iejoin gating (~156, 214-235), has_override seam (181)
• src/giql/expander.py — ExpanderRegistry, ExpansionContext, has_override
• src/giql/targets.py — DuckDBTarget, Capabilities (range_join_strategy)

Dependencies

• Depends on Drop the binned INTERSECTS join path in favor of the naive overlap predicate #167 (the naive-predicate GenericTarget default that this override defers to must exist first).

Out of scope

• Extending IEJoin to newly support shapes it currently declines (Support LEFT/RIGHT JOIN in the DuckDB IEJoin dialect #95-Support 3+ tables with auxiliary non-INTERSECTS joins in the DuckDB IEJoin dialect #111) — this issue migrates and wires deferral; per-shape keep-vs-defer decisions may close some of those separately.
• The GenericTarget naive-predicate default and binned removal — Drop the binned INTERSECTS join path in favor of the naive overlap predicate #167.
• SEMI/ANTI filtering coverage-sweep expander — separate issue.