feat(research): std::walk_forward vocabulary corrected to machinery-true fields (0107 task 1)

folds/in_sample_bars/out_of_sample_bars mapped to nothing
WindowRoller::new accepts; the block now carries in_sample_ms/
out_of_sample_ms/step_ms/mode (WfMode: rolling|anchored) — the
roller's three lengths in the campaign window's own epoch-ms unit
plus both shipped RollModes (#198 decision 2). New intrinsic fault
ZeroWalkForwardLength (path-addressed) refuses zero lengths at the
doc tier, earlier than the roller's NonPositiveLength. Golden
canonical pin, PROCESS_FIXTURE, introspection tables, and the
aura-cli prose/fixture twins moved together; fieldtest corpus
untouched (historical record — stored walk_forward docs get new
content ids by design).

Gates: aura-research 14/14, aura-cli green, workspace build clean.

refs #198

crates/aura-cli/src/research_docs.rs

@@ -97,6 +97,9 @@ fn doc_fault_prose(f: &DocFault) -> String {
         DocFault::GateAfterTerminalStage { stage } => {
             format!("stage {stage}: a gate cannot follow a terminal stage (monte_carlo/generalize)")
         }
+        DocFault::ZeroWalkForwardLength { stage, field } => {
+            format!("pipeline[{stage}]: walk_forward {field} must be > 0")
+        }
         DocFault::EmptyInstruments => "data.instruments is empty".into(),
         DocFault::NoWindow => "data.windows is empty".into(),
         DocFault::BadWindow { index } => {
@@ -401,4 +404,11 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn zero_walk_forward_length_prose_is_path_addressed_and_debug_free() {
+        let prose = doc_fault_prose(&DocFault::ZeroWalkForwardLength { stage: 2, field: "step_ms" });
+        assert_eq!(prose, "pipeline[2]: walk_forward step_ms must be > 0");
+        assert!(!prose.contains("ZeroWalkForwardLength"), "Debug leak: {prose}");
+    }
 }