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.lock

@@ -796,6 +796,16 @@ dependencies = [
  "vcpkg",
 ]
 
+[[package]]
+name = "libyml"
+version = "0.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3302702afa434ffa30847a83305f0a69d6abd74293b6554c18ec85c7ef30c980"
+dependencies = [
+ "anyhow",
+ "version_check",
+]
+
 [[package]]
 name = "line-clipping"
 version = "0.3.7"
@@ -1360,6 +1370,8 @@ dependencies = [
  "rand 0.10.1",
  "ratatui",
  "rusqlite",
+ "serde",
+ "serde_yml",
  "sysinfo",
  "tempfile",
  "thiserror 2.0.18",
@@ -1531,6 +1543,21 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "serde_yml"
+version = "0.0.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59e2dd588bf1597a252c3b920e0143eb99b0f76e4e082f4c92ce34fbc9e71ddd"
+dependencies = [
+ "indexmap",
+ "itoa",
+ "libyml",
+ "memchr",
+ "ryu",
+ "serde",
+ "version_check",
+]
+
 [[package]]
 name = "sha2"
 version = "0.10.9"