spec: 0108 annotator stages (boss-signed) + tier-boundary characterization pin

Cycle 0108, anchored on #200. Encodes the five swarm-triaged decisions
(#200 comment 2026-07-03, all convergent on cited ground, no bounce):
std::monte_carlo bootstraps the stage's incoming R-evidence (per-survivor
after sweep/gates, pooled per-window OOS series after walk_forward) via
the shipped r_bootstrap; std::generalize runs at campaign scope over
per-cell nominees per (strategy, window); both stay terminal annotators
(unanimous); realization widens existing records (StageRealization.bootstrap,
CampaignRunRecord.generalizations, serde-default); v2 executable shape
sweep [gate]* [wf]? [mc]? [generalize]? with static preflight guards.
No document-schema bytes move — every existing content id survives.

Boss-signed via grounding-check PASS (11/11). The first dispatch
BLOCKed on one unpinned current-behaviour assumption ([sweep, mc,
walk_forward] passes the intrinsic tier); the gap was closed by the
green characterization test
validate_process_permits_walk_forward_after_an_annotator (pins the
intrinsic/executor tier boundary), and the fresh re-dispatch ratified
all eleven assumptions.

refs #200

crates/aura-research/src/lib.rs

@@ -1168,6 +1168,26 @@ mod tests {
         assert!(ca.contains(r#""values":[8.0,12.0,16.0]"#));
     }
 
+    /// The intrinsic tier does NOT constrain where a population-transforming
+    /// stage sits relative to the annotators (terminal_seen gates only Gate
+    /// stages): `[sweep, monte_carlo, walk_forward]` is intrinsically valid.
+    /// Execution-shape refusals are the executor preflight's job, one tier up
+    /// — this pin is the boundary between the two tiers.
+    #[test]
+    fn validate_process_permits_walk_forward_after_an_annotator() {
+        let doc = parse_process(
+            r#"{ "format_version": 1, "kind": "process", "name": "wf-after-mc",
+                 "pipeline": [
+                   { "block": "std::sweep", "metric": "sqn", "select": "argmax" },
+                   { "block": "std::monte_carlo", "resamples": 100, "block_len": 5 },
+                   { "block": "std::walk_forward", "in_sample_ms": 4000, "out_of_sample_ms": 1000,
+                     "step_ms": 1000, "mode": "rolling", "metric": "sqn", "select": "argmax" }
+                 ] }"#,
+        )
+        .unwrap();
+        assert_eq!(validate_process(&doc), Vec::new());
+    }
+
     #[test]
     fn validate_process_accepts_the_fixture_and_reports_each_fault() {
         let ok = parse_process(PROCESS_FIXTURE).unwrap();