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.

docs/plans/20260602-test-consolidation.md

@@ -0,0 +1,88 @@
+# Plan: consolidate integration tests into one binary
+
+**Date:** 2026-06-02 · **Status:** done
+
+## Outcome (measured)
+
+| Metric | Before | After |
+|---|---|---|
+| `target/debug/deps` | 1.5 GB | **629 MB** (−58%) |
+| Integration-test binaries | 26 | **2** (`it` + `typing_surface_matrix`) |
+| Clean test build | 22.9 s | 22.4 s |
+| Lib-edit relink | 13.3 s | 12.4 s |
+| Tests | 2151/0/1 | 2151/0/1 (identical) |
+
+cargo auto-detected `tests/it/main.rs` as the `it` target — no
+`[[test]]` entry, no fixups, no warnings.
+
+**Correction to the build-time hypothesis:** the expected relink
+speed-up (≈13 s → ≈5 s) did *not* materialise. Wall-clock is dominated
+by *compiling* (the lib + ~290 deps + the test code's
+codegen/monomorphisation), not by *linking*. Consolidation collapses 26
+link steps into 2 — which is why **disk** drops 58 % (each link emits a
+whole standalone binary) — but compile work is unchanged, so build time
+barely moves. **The real, durable win is disk, not speed.** Minor cost:
+editing a single test file now recompiles the whole `it` module set
+(test-only edits marginally slower; lib edits marginally faster — a
+wash). Kept for the disk benefit, which is exactly the concern that
+prompted it.
+
+## Why
+
+Each top-level `tests/*.rs` is a separate integration-test *crate* →
+its own binary, each statically linking the bundled engine + every
+dependency + debug info. There are **26** of them. With incremental
+disabled (build-hygiene change, commit `42f9553`), an edit to the lib
+forces **all 26 to relink**.
+
+**Measured baseline:**
+- Lib-edit relink (touch `src/app.rs`, `cargo test --no-run`): **13.3 s**
+- Clean build of all test targets (`cargo clean && cargo test --no-run`): **22.9 s**
+- `target/debug/deps`: **1.5 GB**
+
+Collapsing to one binary turns 26 links into 1 — the dominant cost in
+both numbers.
+
+## Approach
+
+The pattern already used by `tests/typing_surface_matrix.rs` (one
+binary that `mod`-includes the `tests/typing_surface/` modules). Apply
+it to the other 25 standalone files.
+
+Feasibility pre-checks (done, all clear):
+- **No top-level integration test uses insta** → moving files causes
+  no snapshot-path churn.
+- **No crate-level `#![…]` attributes** to hoist.
+- **No shared `mod common;` helpers** to relocate; the 25 files are
+  self-contained (`use rdbms_playground::…`), no inter-file `mod`.
+
+## Steps
+
+1. `mkdir tests/it`.
+2. `git mv` the 25 standalone `tests/*.rs` → `tests/it/<name>.rs`
+   (everything except `typing_surface_matrix.rs`, which stays as its
+   own already-consolidated binary).
+3. Create `tests/it/main.rs` = `mod <name>;` for all 25.
+4. Confirm cargo builds it as one target (`tests/it/main.rs`
+   auto-detection; add an explicit `[[test]] name="it"` only if not).
+5. Fix anything the combined build surfaces (expected minimal): a
+   stray `use crate::` self-ref, or a clippy `dead_code`/unused-import
+   the per-crate split had hidden.
+
+**End state: 2 integration-test binaries (`it` + `typing_surface_matrix`)
+instead of 26.**
+
+## Verification / go-no-go gate
+
+- Full `cargo test` → identical **2151 / 0 / 1**.
+- `cargo clippy --all-targets -- -D warnings` clean.
+- Re-measure relink + clean build; record before/after in the commit.
+- If the after-numbers aren't materially better, **revert** (all
+  `git mv`, trivially reversible).
+
+## Tradeoff (accepted)
+
+Lose per-crate isolation: a compile error fails the whole `it` binary,
+and the 25 files share one crate namespace (module-scoped — no name
+clashes). Negligible for a solo teaching project; the build-speed win
+is the priority.