feat: ADR-0006 §8 step 6 — .snapshots/ gitignore + export + cleanup

The undo ring is local working state, handled at all three
project-file seams (R13):
- .gitignore template ignores /.snapshots/
- export excludes .snapshots/ (like playground.db / history.log)
- safely_delete_temp_project allowlists .snapshots/ so a temp that
  was modified then undone back to empty stays auto-deletable
- undo::SNAPSHOTS_DIR is now a pub const referenced by all three
- tests: gitignore content, export exclusion, cleanup allowlist

1693 passed / 0 failed / 1 ignored; clippy clean.

src/archive.rs

@@ -43,6 +43,9 @@ const EXPORT_EXCLUDED_NAMES: &[&str] = &[
     PLAYGROUND_DB,
     HISTORY_LOG,
     ".rdbms-playground.lock",
+    // Undo snapshot ring (ADR-0006 Amendment 1): local working
+    // state, never shared.
+    crate::undo::SNAPSHOTS_DIR,
 ];
 
 /// Maximum auto-suffix attempts when resolving a colliding
@@ -306,6 +309,7 @@ fn add_directory_recursive(
 /// - `playground.db` (always derived; ADR-0007 / ADR-0015 §11)
 /// - `history.log` (ADR-0007 amendment 1: user-private)
 /// - `.rdbms-playground.lock` (per-process)
+/// - `.snapshots/` (undo ring; local working state, ADR-0006 Am. 1)
 /// - `*.tmp` (atomic-write staging files)
 /// - `project.yaml.v*.bak` (migration backups; recipient
 ///   doesn't need our local recovery aids)
@@ -524,6 +528,18 @@ mod tests {
         fs::write(p.join(GITIGNORE), "/playground.db\n").unwrap();
         // Stray atomic-write staging file — must be excluded.
         fs::write(p.join("project.yaml.tmp"), "stale").unwrap();
+        // Undo snapshot ring — local working state, must be excluded.
+        fs::create_dir_all(p.join(crate::undo::SNAPSHOTS_DIR).join("0")).unwrap();
+        fs::write(
+            p.join(crate::undo::SNAPSHOTS_DIR).join("index.yaml"),
+            "next_id: 1\nundo: []\nredo: []\n",
+        )
+        .unwrap();
+        fs::write(
+            p.join(crate::undo::SNAPSHOTS_DIR).join("0").join(PLAYGROUND_DB),
+            [0u8; 16],
+        )
+        .unwrap();
         p
     }
 
@@ -548,6 +564,11 @@ mod tests {
         assert!(!names.iter().any(|n| n.contains("history.log")));
         assert!(!names.iter().any(|n| n.contains(".lock")));
         assert!(!names.iter().any(|n| n.ends_with(".tmp")));
+        // Undo snapshot ring is local working state — never shared.
+        assert!(
+            !names.iter().any(|n| n.contains(".snapshots")),
+            "snapshots leaked into export: {names:?}"
+        );
         // .gitignore IS included (sensible default for the recipient).
         assert!(names.iter().any(|n| n == "MyProject/.gitignore"));
     }

src/project/mod.rs

@@ -416,10 +416,12 @@ impl Project {
         write_if_missing(&path.join(PROJECT_YAML), &yaml)?;
 
         // .gitignore template (ADR-0015 §11). Excludes the
-        // derived `.db`, the per-process lock, and migration
-        // backups. `history.log` is intentionally NOT ignored
+        // derived `.db`, the per-process lock, migration backups,
+        // and the undo snapshot ring (ADR-0006 Amendment 1 — local
+        // working state). `history.log` is intentionally NOT ignored
         // (ADR-0007 amendment 1: per-user choice).
-        let gitignore = "/playground.db\n/.rdbms-playground.lock\n/project.yaml.v*.bak\n";
+        let gitignore =
+            "/playground.db\n/.rdbms-playground.lock\n/project.yaml.v*.bak\n/.snapshots/\n";
         write_if_missing(&path.join(GITIGNORE), gitignore)?;
 
         Ok(())
@@ -502,6 +504,10 @@ const ALLOWED_PROJECT_ENTRIES: &[&str] = &[
     PLAYGROUND_DB,
     GITIGNORE,
     ".rdbms-playground.lock",
+    // Undo snapshot ring (ADR-0006 Amendment 1): a temp that was
+    // modified then undone back to empty can still carry this, and
+    // must remain auto-deletable.
+    crate::undo::SNAPSHOTS_DIR,
 ];
 
 /// Reasons `safely_delete_temp_project` refuses to delete a
@@ -841,6 +847,7 @@ mod tests {
         let gi = fs::read_to_string(path.join(GITIGNORE)).unwrap();
         assert!(gi.contains("/playground.db"));
         assert!(gi.contains("/.rdbms-playground.lock"));
+        assert!(gi.contains("/.snapshots/"), "undo ring should be ignored");
         assert!(!gi.contains("history.log"), "history.log should NOT be ignored");
     }
 

src/undo.rs

@@ -49,7 +49,11 @@ use crate::project::{DATA_DIR, PLAYGROUND_DB, PROJECT_YAML};
 pub const DEFAULT_RING_CAPACITY: usize = 50;
 
 /// Directory under the project that holds the ring.
-const SNAPSHOTS_DIR: &str = ".snapshots";
+///
+/// Public so the `.gitignore` template, the export exclusion, and
+/// the temp-project cleanup allowlist all reference the one
+/// canonical name (ADR-0006 Amendment 1).
+pub const SNAPSHOTS_DIR: &str = ".snapshots";
 /// In-flight snapshot directory (one at a time).
 const STAGING_DIR: &str = ".staging";
 /// The ring index file.