fix(app): hold input until the schema-cache refresh lands (#39)

A simple-mode Form-B insert (`insert into T values (…)`) submitted faster
than the post-DDL schema-cache refresh was validated against the stale
cache and wrongly rejected as "trying to write SQL?". The cache is
refreshed asynchronously after the worker applies a command, posting
`SchemaCacheRefreshed` on the same FIFO channel as key events; under
faster-than-human input the next Enter is processed before the refresh.

Add a submission gate: `dispatch_dsl` arms `awaiting_schema_refresh` on
every `ExecuteDsl` dispatch, holds further DSL submissions in a
`held_submissions` FIFO while armed, and the `SchemaCacheRefreshed`
handler drains them against the fresh cache in submission order.
App-lifecycle commands bypass the gate. Arm-on-every-dispatch keeps
exactly one DSL command in flight, so the boolean is provably correct.
Interactive-only: the replay/batch path already re-snapshots the schema
synchronously per line. No interactive-user impact; held input is never
lost (a refresh always follows a dispatch).

Tests (both verified red→green): a Tier-1 deterministic ordering test and
a Tier-4 PTY back-to-back regression (the unpaced inverse of flow 3).
Records the design as ADR-0022 Amendment 8.

tests/e2e_pty.rs

@@ -377,6 +377,33 @@ fn undo_after_drop_table_restores_it() {
     app.quit();
 }
 
+/// Flow 5 — issue #39 regression: commands submitted **back-to-back**, with
+/// no wait between them, must still execute correctly. This is the inverse of
+/// flow 3 (which paces each command on purpose). Pre-fix, a Form-B insert sent
+/// immediately after `add column` was validated against the *stale* schema
+/// cache — the worker hadn't yet refreshed it — and wrongly rejected as
+/// "trying to write SQL?", so the row never landed. The schema-refresh gate
+/// (`App::awaiting_schema_refresh`) now holds each submission until the prior
+/// command's refresh arrives, making the outcome independent of input speed.
+#[test]
+fn back_to_back_insert_after_ddl_still_succeeds() {
+    let mut app = PtyApp::launch(&[]);
+    app.wait_for_no_tables(); // fresh project
+
+    // Fire all three with no readiness wait in between — the faster-than-human
+    // input that triggered issue #39 (paste / script / unpaced driver).
+    app.submit("create table Customers with pk id(serial)");
+    app.submit("add column to Customers: Name (text)");
+    app.submit("insert into Customers values ('Alice')");
+
+    // The insert's OWN success echo (value + ✓) — proof the row reached the
+    // database, not the "trying to write SQL?" rejection. If the gate
+    // regressed, the insert would misparse against the stale schema and this
+    // would time out.
+    app.wait_for("('Alice') ✓");
+    app.quit();
+}
+
 // ===================== NFR perf (measured, generous) ===================
 //
 // These run against the DEBUG binary, so the bounds are loose