ci(bench): erd_open_200 scenario times out in app_perf_bench; make bench failures exit non-zero #1546

Problem

benchmark/app_perf_bench.dart (added in #1173) fails in the erd_open_200 scenario on current dev:

BENCH error: Bad state: timeout waiting for diagram of 200 tables

The scenario runs a 200-table DDL script in a new tab, taps open_diagram_tab, and waits up to 20 s for the card with key erd_table_erd_t199. The key still exists (lib/features/erd/widgets/erd_table_card.dart).

What was found

Re-running with only this scenario and dumping the visible texts on timeout showed the UI on the connection's Overview tab with Tables 3 (the demo's own tables), and no erd_table_* cards. So either the DDL script did not create the tables (multi-statement run in one tab) or the Diagram tab was never opened. It is not shown to be an ERD performance problem.

Because the scenario aborts, everything after it in the bench is unchecked, and the ERD scenario guards nothing.

Scope

• Find out which of the two it is (assert the table count after the DDL run; assert the Diagram tab is the active one before waiting).
• Fix the scenario so it opens the diagram and reports build/raster for the 200-table layout and first paint.
• Make the bench fail loudly (non-zero exit) when a scenario times out, instead of printing BENCH error and exiting 0.
• Optional: a budget check for run_heavy_query (max build) so a regression fails the run.

Acceptance

• erd_open_200 runs and prints build/raster numbers.
• A scenario timeout makes the bench exit non-zero.