feat(fynd-core): expose BlockStepController for gated block ingestion

[kayibal]

Summary

• Adds FyndBuilder::build_with_step_controller() — an async builder method that returns (Solver, BlockStepController). The Tycho feed runs in step-controlled mode: each incoming block is buffered and held until the caller calls controller.trigger_next_block().
• Re-exports BlockStepController from fynd_core (alongside PendingBlockProcessor) so consumers don't need to import from tycho_simulation directly.
• Adds SolverBuildError::StepControllerChannelClosed for the case where the feed task panics before delivering the controller.

The implementation follows the same pattern as build_with_pending: a oneshot channel delivers the BlockStepController from TychoFeed::run_with_step_controller back to the builder before the event loop starts.

Upstream: propeller-heads/tycho#1067 (merged).

Usage

let (solver, controller) = FyndBuilder::new(...)
    .algorithm("most_liquid")
    .build_with_step_controller()
    .await?;

// Peek at the next block before processing it
let block = controller.peek_next_block().await;

// Release it for the solvers
controller.trigger_next_block()?;

Dependency

Blocked on the next tycho-simulation release after 0.302.0 that includes BlockStepController. Once published:

1. Update tycho-simulation = ">=<NEW_VERSION>" in Cargo.toml (workspace dependencies).
2. Run cargo check --workspace to update Cargo.lock.
3. CI will pass.

Test plan

• Wait for next tycho-simulation crates.io release with BlockStepController
• Update version constraint and verify cargo check --workspace passes
• Run ./check.sh to confirm no regressions

🤖 Generated with Claude Code