Constant PROJECTION_PLACEMENT_KEY 

pub const PROJECTION_PLACEMENT_KEY: &str = "__projection_placement";

ctx.meta.runtime key under which the Blueprint-wide crate::core::projection_placement::ProjectionPlacement resolver (JSON-serialized, GH #27 follow-up to #23) is stashed by crate::middleware::agent_context::AgentContextMiddleware — the read-back source for the spawn-time in-flight projection pointer (crates/mlua-swarm-server/src/operator_ws/session.rs’s append_projection_pointer), which has no direct Engine handle to call Engine::projection_placement_for with. Absent (or undeserializable) falls back to ProjectionPlacement::default() (byte-compat).