style: fmt + clippy cleanup after Phase B

cargo fmt regroups, plus three Phase-B clippy fixes:
- F-P-004: #[allow(clippy::type_complexity)] on the invoke_ast_cache Mutex.
- F-P-008: hoist `futures::stream::{self, TryStreamExt}` to the top-of-
  file imports to satisfy clippy::items_after_statements.
- F-P-005: rename `_legacy_offset` to `legacy_offset` + #[allow(dead_code)]
  to satisfy clippy's "field is pub but `_`-prefixed" check.

No behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/executor-core/src/engine.rs

@@ -58,6 +58,7 @@ pub struct Engine {
     /// callee on every invoke. Independent of the orchestrator-core
     /// `LocalExecutorClient` AST cache — that one caches HTTP-path
     /// dispatch; this one caches function-call dispatch.
+    #[allow(clippy::type_complexity)]
     invoke_ast_cache: Mutex<HashMap<ScriptId, (DateTime<Utc>, Arc<AST>)>>,
 }