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.

Cargo.toml

@@ -20,6 +20,8 @@ gethostname = "1.1.0"
 rand = "0.10.1"
 ratatui = "0.30.0"
 rusqlite = { version = "0.39.0", features = ["bundled"] }
+serde = { version = "1.0.228", features = ["derive"] }
+serde_yml = "0.0.12"
 sysinfo = { version = "0.39.0", default-features = false, features = ["system"] }
 thiserror = "2.0.18"
 tokio = { version = "1.52.2", features = ["full"] }