Iteration 3: existence-only load + rebuild from text on missing .db

When the runtime opens a project whose playground.db is missing,
it now rebuilds the database from project.yaml + data/<table>.csv
per ADR-0015 §7. The rebuild path:

1. Parses project.yaml (serde_yml). Unknown versions / types /
   actions surface as PersistenceFatal.
2. Recreates each user table with FK constraints inline
   (PRAGMA foreign_keys=OFF), then populates the column-type,
   relationship, and project metadata tables.
3. Loads each table's CSV via a hand-rolled reader that
   preserves the NULL-vs-empty distinction (the csv crate
   doesn't expose whether a field was quoted; ours does).
4. Runs PRAGMA foreign_key_check before commit; any violation
   aborts.
5. Restores foreign_keys=ON regardless of success.

Row-level failures get DbError::RebuildRowFailed with row
number, file, table, and a friendly per-type detail. They land
in the runtime as a fatal stderr message ("unable to load row N
from `data/T.csv` into table `T`: ...") before the alternate
screen is entered.

created_at from project.yaml overwrites the configure-time
placeholder so timestamps round-trip stably.

Tests: 307 passing (267 lib + 9 + 5 new + 9 + 17), 0 failing,
0 skipped. Clippy clean with nursery lints.

src/runtime.rs

@@ -47,8 +47,29 @@ pub async fn run(args: Args) -> Result<()> {
     let db_path = project.db_path();
     let display_name = project.display_name().to_string();
     let persistence = crate::persistence::Persistence::new(project.path().to_path_buf());
+    // Capture whether the .db file existed BEFORE we open it —
+    // sqlite creates it on connect, so this is the only honest
+    // signal that we need to rebuild from text (ADR-0015 §7).
+    let db_existed = db_path.exists();
     let database = Database::open_with_persistence(db_path.as_path(), persistence)
         .context("open database")?;
+    if !db_existed
+        && let Err(e) = database.rebuild_from_text(project.path().to_path_buf()).await
+    {
+        // The terminal is still in cooked mode here (we haven't
+        // entered the alternate screen yet), so writing to
+        // stderr lands directly in the user's shell. Drop the
+        // project to release the lock first.
+        drop(project);
+        if matches!(
+            e,
+            DbError::PersistenceFatal { .. } | DbError::RebuildRowFailed { .. }
+        ) {
+            eprintln!("rdbms-playground: {}", e.friendly_message());
+            return Ok(());
+        }
+        return Err(anyhow::anyhow!(e.friendly_message())).context("rebuild from text");
+    }
 
     let mut terminal = setup_terminal().context("setup terminal")?;
     let result = run_loop(&mut terminal, args.theme, database, display_name).await;