Model: flatten GlobalLineage into top-level nodes/edges with statementIds

[melonamin]

Split out from #17.

Problem

AnalyzeResult currently carries four parallel collections: StatementLineage.nodes, StatementLineage.edges, GlobalLineage.nodes, GlobalLineage.edges (crates/flowscope-core/src/types/response.rs:82-170, :521-629). Consumers have to reconcile per-statement vs global IDs, and features like multi-file analysis or lazy expansion would need yet another list pair.

Proposal

• Introduce flat nodes: Vec<Node> and edges: Vec<Edge> at the top level of AnalyzeResult.
• Extend Node with statementIds: Vec<usize> (and optional sourceName) so statement grouping is derived dynamically.
• Keep statements: Vec<StatementMeta> for per-statement metadata (type, span, complexity) but drop the embedded graph.
• Fold GlobalNode/GlobalEdge fields (canonicalName, statementRefs, producerStatement, consumerStatement) into Node/Edge.
• EdgeType::CrossStatement stays — assigned at build time when the edge bridges statement groups.

Non-goals

No schema/resolution changes. No UI/TS binding rewrite beyond adapter shims.

Acceptance

• Rust + TS types updated; single source of truth in response.rs / packages/core/src/types.ts.
• Existing consumers (VS Code extension, app) keep working via a thin adapter during one release cycle.
• JSON snapshot tests updated and cargo test --workspace green.

Foundational — unblocks downstream graph/navigation/lazy-loading work.