docs: archive 17 stale planning docs to docs/historical/ (REQ-074 Phase 2a)

docs/what-is-rivet.md

@@ -382,8 +382,9 @@ server-side-validated before it lands.
 - **Polarion / ReqIF fidelity audit.**
   [docs/design/polarion-reqif-fidelity.md](design/polarion-reqif-fidelity.md).
 - **Roadmap.** [docs/roadmap.md](roadmap.md).
-- **Audit report.** [docs/audit-report.md](audit-report.md) — state
-  of doc-vs-reality after PR #171's audit.
+- **Audit report.** [docs/historical/audit-report.md](historical/audit-report.md)
+  — archived doc-vs-reality snapshot (2026-03-09). Superseded by
+  [docs/research/2026-05-19-docs-audit.md](research/2026-05-19-docs-audit.md).
 
 ---
 

rivet-core/src/doc_check.rs

@@ -134,10 +134,15 @@
 
 impl DocFile {
     pub fn new(rel_path: PathBuf, content: String) -> Self {
+        // `plans/` and `design/` are forward-looking; `historical/` is
+        // frozen archive (REQ-073/074). All three legitimately reference
+        // subcommands / artifact counts / IDs that have since changed,
+        // so they opt out of the existence-based invariants.
         let is_design_doc = content.contains("rivet-docs-check: design-doc-aspirational-ok")
-            || rel_path
-                .components()
-                .any(|c| c.as_os_str() == "plans" || c.as_os_str() == "design");
+            || rel_path.components().any(|c| {
+                let s = c.as_os_str();
+                s == "plans" || s == "design" || s == "historical"
+            });
         Self {
             rel_path,
             content,
@@ -1169,10 +1174,10 @@
     // that is actually part of a longer hex run.
     if let Some((head, tail)) = id.rsplit_once('-') {
         if tail.len() >= 4
             && tail.chars().all(|c| c.is_ascii_hexdigit())
             && tail.chars().any(|c| c.is_ascii_alphabetic())
             // at least one letter ==> not a pure decimal id
             && head.chars().all(|c| c.is_ascii_uppercase() || c == '-')
         {
             return true;
         }
@@ -1223,13 +1228,13 @@
 
 impl DocInvariant for MigrationConflict {
     fn name(&self) -> &'static str {
         "MigrationConflict"
     }
 
     fn check(&self, ctx: &DocCheckContext<'_>) -> Vec<Violation> {
         let mut out = Vec::new();
         let root = ctx.project_root.join("artifacts");
         if !root.is_dir() {
             return out;
         }
         let mut files = Vec::new();
@@ -1256,7 +1261,7 @@
                 if let Some(k) = kind {
                     out.push(Violation {
                         file: rel.clone(),
                         line: idx + 1,
                         invariant: "MigrationConflict".to_string(),
                         claim: format!("artifact YAML contains migration conflict marker ({k})"),
                         reality: "run `rivet schema migrate --status` for context, then \
@@ -1282,7 +1287,7 @@
         } else if p
             .extension()
             .and_then(|e| e.to_str())
             .is_some_and(|e| e == "yaml" || e == "yml")
         {
             out.push(p);
         }
@@ -1586,6 +1591,27 @@
         assert_eq!(v[0].file, PathBuf::from("README.md"));
     }
 
+    /// REQ-073/074: a file under `docs/historical/` is a frozen archive
+    /// snapshot and opts out of the existence-based invariants — same as
+    /// `plans/` and `design/` — without needing the explicit marker.
+    ///
+    /// rivet: verifies REQ-074
+    #[test]
+    fn historical_dir_is_design_doc_without_marker() {
+        assert!(
+            doc("docs/historical/old-plan.md", "Run `rivet discover`.").is_design_doc,
+            "docs/historical/ must be exempt from existence invariants"
+        );
+        assert!(
+            doc("docs/plans/p.md", "x").is_design_doc,
+            "docs/plans/ exemption must still hold"
+        );
+        assert!(
+            !doc("docs/getting-started.md", "x").is_design_doc,
+            "a normal reference doc must NOT be auto-exempt"
+        );
+    }
+
     #[test]
     fn subcommand_references_skip_design_doc() {
         let content = "<!-- rivet-docs-check: design-doc-aspirational-ok -->\n\