make test-rewatch is documented as the Rust build system's test command but tests the OCaml one #8969

Found by differential semantic drift analysis from a7721303 to b123b7c3.

What is broken

The Rust build system's docs give make test-rewatch as its test command, but that target runs the suite against the OCaml build system.

• rewatch/README.md L39-L40 lists make test-rewatch under "Building and testing" for the Rust implementation, and rewatch/AGENTS.md L75-L81 describes it as rewatch's integration tests.
• Makefile L188-L189 runs ./rewatch/tests/suite.sh $(RESCRIPT_EXE), and L81 sets RESCRIPT_EXE = $(BIN_DIR)/rescript.exe, which compiler/sync/dune promotes from rewatch-ocaml/rescript_ocaml.exe. The Rust binary is rescript-rust.exe (L82), and test-rewatch depends on lib, which does not build the rewatch target.
• CI runs the suite on rescript-rust.exe only for linux-arm64 (ci.yml L273-L276).

So someone changing rewatch/src/*.rs and running make test-rewatch, as the Rust docs say, neither rebuilds nor tests the Rust binary.

Introduced by #8780, which made rescript.exe the OCaml build system; before it the same recipe tested the Rust binary.

Possible cause (speculation only)

A guess, not established: the port updated the OCaml build system's docs and the root instructions, and the Rust docs, written when there was one implementation, were not revisited.