Relocate CLUSTER and MERGE into the operator-expander registry — Closes #144

[conradbzura]

Summary

Migrate the last two operators, CLUSTER and MERGE, off the pre-pass transformer chain and onto the ExpandOperators registry (epic #137), so every GIQL operator now dispatches through one mechanism. CLUSTER and MERGE are whole-query rewrites, so each expander navigates to its enclosing SELECT, restructures a detached copy, and transplants the result back onto the original SELECT in place — returning the operator node unchanged (a no-op replace). This root-preserving transplant is required because the canonical form puts the operator at the root SELECT, which has no parent to replace through. The pass's deepest-first walk subsumes the transformers' manual recursion into CTEs and subqueries, and the standalone ClusterTransformer / MergeTransformer classes and their pre-pass calls are removed.

Emitted SQL is semantically identical to the legacy output and now deterministic; the only intended byte difference is the injected lag_calc column order, which the legacy output left hash-seed-dependent. Because the in-place rewrite copies the enclosing WHERE/HAVING into the new inner subquery, the expander re-runs the pass over the restructured SELECT (threading the active registry through ExpansionContext) so a sibling operator carried into that subquery — a spatial predicate or DISTANCE in a filtered cluster/merge — is expanded rather than leaking to the generator.

A few shapes the legacy pipeline accepted only to emit non-executable or invalid SQL now raise a clear ValueError instead: CLUSTER and MERGE in one SELECT, multiple CLUSTER expressions in one SELECT (as multiple MERGE already did), and a CLUSTER/MERGE nested inside a larger projection expression. The MERGE GROUP BY chromosome term is now quoted, so a reserved-word custom chrom column emits valid SQL. No working query is affected.

Two non-blocking follow-ups are deferred: extracting the shared CLUSTER/MERGE toolkit into a neutral module (#163) and resolving genomic columns over a derived-table FROM (#164).

Closes #144

Proposed changes

• New giql/expanders/cluster.py and giql/expanders/merge.py — generic (GenericTarget) expanders registered for GIQLCluster / GIQLMerge, porting the legacy two-level lag_calc and clustered-aggregation rewrites. MERGE composes CLUSTER's expand_cluster_query. Shared helpers (genomic_columns, transplant, find_projected, extract_stranded, reject_cluster_merge_mix, require_top_level_projection, GenomicColumns) form the cross-operator toolkit.
• giql/expander.py — ExpansionContext now carries the active registry, letting a whole-query rewrite re-enter expand_operators over the SELECT it just restructured.
• giql/expressions.py — GIQL_EXPAND = True on GIQLCluster / GIQLMerge; they deliberately skip canonicalization and carry an empty resolution.
• giql/resolver.py — GIQLCluster / GIQLMerge added to the _OPERATORS roster; module scope note reconciled to the post-migration state.
• giql/transformer.py / giql/transpile.py — ClusterTransformer / MergeTransformer and their pre-pass calls removed (−661 lines in transformer.py).
• docs/dialect/aggregation-operators.rst — document the CLUSTER+MERGE-in-one-SELECT ValueError and the separate-queries workaround.

Test cases

#	Test Suite	Given	When	Then	Coverage Target
1	TestClusterExpander	A CLUSTER nested in a subquery, CTE, or UNION branch	Transpiling	It expands to the lag_calc form with no leaked operator	Pass-walk recursion
2	TestClusterExpander	An aliased or bare CLUSTER filtered by a spatial predicate in WHERE	Transpiling	The predicate copied into lag_calc is itself expanded — no leak	B1 regression
3	TestClusterExpander	A CLUSTER nested in a projection expression	Transpiling	It raises a top-level-projection ValueError	A16 guard
4	TestClusterExpander	Two CLUSTER expressions in one SELECT	Transpiling	It raises a multiple-CLUSTER ValueError	A15 guard
5	TestClusterExpander	An explicit-projection CLUSTER under two PYTHONHASHSEED values	Transpiling in subprocesses	Output is byte-identical	Determinism
6	TestMergeExpander	An aliased or bare MERGE filtered by a spatial predicate in WHERE	Transpiling	The copied predicate is expanded — no leak	B1 regression
7	TestMergeExpander	A MERGE over a Table whose chrom column is a reserved word	Transpiling	The GROUP BY chrom term is quoted	A13 fix
8	TestMergeExpander	CLUSTER and MERGE in one SELECT, or multiple/nested MERGE	Transpiling	It raises the matching ValueError	Guard coverage
9	TestExpander*	All nine operators migrated to the registry	Running the pass	No operator remains unmigrated; CLUSTER/MERGE rewrite in place	Migration completeness