feat(slice): SqlObjectGraph — unified object-graph IR with cascade safety and ownership-based change pruning

[pyramation]

Summary

Stage 4 foundation (constructive-planning #1322/#1323), stacked on #1535: the semantic object-graph IR over parsed SqlPrograms. This is the single graph that exclusion, routing, and plan pruning are meant to share — replacing survivor-counting and flattened whole-module classification with graph queries.

New pgpm/slice/src/object-graph.ts:

buildObjectGraph(programs: Map<changeName, SqlProgram>): SqlObjectGraph
// nodes: objects → { ref, createdBy: StatementId[] }
// edges: statement → object, kind: 'reference' | 'body' | 'fk'
// incoming: object key → in-edges

objectsInSchemas(graph, schemas)   // subsystem selection by schema
danglingEdges(graph, dropped)      // cascade safety: in-edges to dropped objects
                                   // whose source statement is not itself dropped
prunablePrograms(graph, dropped)   // whole changes owned by the dropped set —
                                   // read from ownership, not counted from survivors

Semantics worth noting:

• Cascade safety falls out of the graph: auth.uid()'s body reference to auth.users does not dangle when both are dropped (its source statement creates a dropped object), while a surviving FK / policy accessor into the same table does — tested with exactly this shape.
• Change pruning is ownership-based: a program is prunable iff every object-creating statement creates only dropped objects; mixed programs and pure-grant programs are never pruned. This replaces the dropped>0 && survivors==0 counting heuristic in the apply path (rewiring materialize onto these queries is the follow-up).
• Statement identity is {program, statement} against the shared SqlProgram from feat(transform): SqlProgram — statement-level program AST with dirty-aware verbatim/deparse emission #1535, so the graph, exclusion analysis, and emission all reference the same single parse.

Pure/I-O-free; 6 new tests (54 total in slice green).

Link to Devin session: https://app.devin.ai/sessions/025fb88043964fdbb335ac5e39df2478
Requested by: @pyramation

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