feat: detect self-referencing targets across pipeline statements

[mingjerli]

Summary

• Self-read detection: When a multi-statement pipeline writes and reads the same table (e.g., SCD2 MERGE + INSERT on dim_customer), clgraph now creates query-scoped self-read nodes instead of collapsing both references onto a single node
• Cycle-safe dependencies: _build_query_dependencies and _build_table_dependencies exclude self-dependencies, preventing topological sort cycles
• Cross-query wiring: Column-granular, topo-ordered edges connect prior statement output to self-read input nodes, with edge_role and statement_order annotations on all edges
• Patterns covered: MERGE+INSERT (SCD2), DELETE+INSERT, single-statement INSERT INTO t SELECT ... FROM t, aliased self-references

Closes Gap 4 from the CDC/SCD pipeline gap analysis (docs/superpowers/specs/2026-04-13-gap4-self-referencing-target-design.md).

Files Changed

File	Change
models.py	Added DELETE to SQLOperation, self_referenced_tables/self_ref_aliases to ParsedQuery, statement_order/edge_role to ColumnEdge
multi_query.py	Refactored _extract_source_tables to use AST node identity for target-slot detection; added exp.Delete handling
table.py	Self-exclusion guards in _build_query_dependencies and _build_table_dependencies
pipeline_lineage_builder.py	Self-read node naming, _is_self_read_column helper, _add_self_read_cross_query_edges method, edge annotations
pipeline.py	Added get_self_read_columns(), fixed get_column() to prefer output layer
test_cdc_scd_pipeline.py	23 test methods covering all 16 spec test cases

Test plan

• All 23 new tests pass (tests/test_cdc_scd_pipeline.py)
• All 1425 existing tests pass (0 regressions)
• Pre-commit hooks pass (ruff format, ruff lint)
• CI pipeline passes