Iteration 4a: rebuild command with confirmation modal

Adds the explicit `rebuild` app-level command (ADR-0015 §7, §11)
and a modal UI infrastructure to host its confirmation dialog.
Typing `rebuild` emits Action::PrepareRebuild; the runtime reads
project.yaml + data/ to compute a summary ("3 tables and 47 rows
will be reconstructed; the existing playground.db will be
replaced") and posts AppEvent::RebuildPrepared, which opens the
modal. Y confirms, N/Esc cancels. While the modal is open,
normal input is gated.

The worker's do_rebuild_from_text now wipes existing user tables
and metadata before reloading from text, so it works on both
fresh and populated databases. Source text is plumbed through
rebuild_from_text so the explicit rebuild logs to history.log
while the silent on-load rebuild from Iteration 3 stays silent.

Modal infrastructure (App.modal field + key routing + centered
overlay rendering + word-wrap) is reused by Iteration 4b's save
/ save as / load / new flows.

Tests: 314 passing (268 lib + 9 + 5 + 6 new + 9 + 17),
0 failing, 0 skipped. Clippy clean.

src/event.rs

@@ -55,4 +55,21 @@ pub enum AppEvent {
         path: std::path::PathBuf,
         message: String,
     },
+    /// Runtime has computed the rebuild summary from
+    /// `project.yaml` + `data/` and is ready for the user to
+    /// confirm. App opens the confirmation modal.
+    RebuildPrepared {
+        summary: String,
+    },
+    /// Rebuild completed successfully. App closes the modal,
+    /// surfaces a friendly outcome message, and refreshes the
+    /// table list.
+    RebuildSucceeded {
+        summary: String,
+    },
+    /// Rebuild failed in a non-fatal way (e.g., user-visible
+    /// constraint problem) — surfaced like other DSL failures.
+    RebuildFailed {
+        error: String,
+    },
 }