Connect burn flow across turns or narrow the contract

[willwashburn]

Problem

burn flow --session <id> is documented as an inference-flow DAG for a session, but the graph currently resets main-rail sequencing at each turn. The builder starts each turn with prev_main_id = None, so default edges only connect nodes within the same turn. The checked-in 3-turn flow snapshot has no edges from turn 0 to turn 1 or turn 1 to turn 2.

That makes the rendered Mermaid and SVG look like separate per-turn skeletons rather than a session flow.

Relevant files

• crates/relayburn-sdk/src/analyze/flow_graph.rs
• crates/relayburn-cli/src/commands/flow.rs
• tests/fixtures/cli-golden/snapshots/flow-json.stdout.txt
• tests/fixtures/cli-golden/snapshots/flow-mermaid.stdout.txt
• crates/relayburn-cli/tests/flow_svg.rs

Suggested scope

Prefer the simpler behavior: connect the previous main-rail terminal node to the next turn main-rail entry with a default edge, then keep dispatch and return edges as the cross-rail layer.

If the intended contract is intentionally per-turn only, rename or document it that way and remove the DAG/session-flow language.

Definition of done

• Multi-turn sessions have deterministic main-rail continuity in JSON, Mermaid, and SVG output, or the command contract is narrowed clearly.
• Add an SDK unit test for two plain turns with a cross-turn default edge.
• Refresh golden JSON/Mermaid/SVG fixtures if the connected graph is the chosen behavior.
• Verify cargo test --workspace and the flow SVG snapshot test.