test: consolidate 25 integration crates into one it binary

Each top-level tests/*.rs was its own crate → its own binary, each
statically linking the bundled engine + every dep. 26 of them, so an
edit to the lib relinked all 26. Moved the 25 standalone files into
tests/it/ under one tests/it/main.rs (the pattern typing_surface
already uses); cargo auto-detects it as the `it` target. End state: 2
integration-test binaries instead of 26.

Result: target/debug/deps 1.5 GB → 629 MB (-58%). Build time barely
moved (clean 22.9s→22.4s, lib-edit relink 13.3s→12.4s) — wall-clock is
dominated by compiling, not linking, so this is a disk win, not a speed
win (see docs/plans/20260602-test-consolidation.md). Tests unchanged at
2151/0/1; clippy clean; no fixups needed. typing_surface_matrix stays
its own already-consolidated binary.

Tradeoff: the 25 files now share one crate (a compile error fails the
whole `it` binary; module-scoped namespaces, no clashes) — negligible
for a solo project.

tests/it/main.rs

@@ -0,0 +1,34 @@
+//! Consolidated integration-test binary (see
+//! `docs/plans/20260602-test-consolidation.md`).
+//!
+//! Each former top-level `tests/*.rs` is now a module here, so the
+//! whole suite links into **one** binary instead of 26 — turning an
+//! edit-the-lib-then-test cycle from 26 separate links into one.
+//! `tests/typing_surface_matrix.rs` stays a separate binary (it is
+//! already a consolidated `mod`-based target).
+
+mod case_insensitive_names;
+mod column_op_guards;
+mod engine_vocabulary_audit;
+mod friendly_enrichment;
+mod iteration2_persistence;
+mod iteration3_rebuild;
+mod iteration4a_rebuild_command;
+mod iteration4b_lifecycle_commands;
+mod iteration5_export_import;
+mod iteration6_resume_history;
+mod parse_error_pedagogy;
+mod project_lifecycle;
+mod replay_command;
+mod sql_alter_table;
+mod sql_create_index;
+mod sql_create_table;
+mod sql_delete;
+mod sql_dml_e2e;
+mod sql_drop_index;
+mod sql_drop_table;
+mod sql_insert;
+mod sql_select;
+mod sql_update;
+mod undo_snapshots;
+mod walking_skeleton;