Make wasm_of_ocaml-compiler an optional dependency

[hhugo]

What

Stop requiring wasm_of_ocaml-compiler to use eliom, make the distillery templates' WebAssembly build opt-in on the toolchain, and add CI that actually builds generated projects (JS-only and wasm).

1. wasm_of_ocaml-compiler is not a dependency

eliom links no wasm_of_ocaml library and never invokes the compiler at build time — eliom_client.wat is shipped as a runtime asset, not compiled, so dune build -p eliom builds and installs identically with or without it. It is only a tool a downstream app uses to compile its client to WebAssembly. So it is removed from the opam metadata entirely (neither depends nor depopts, since a depopt would wrongly imply eliom's build varies with its presence).

Consequently the x-ci-accept-failures entry for centos-10 / opensuse-* (added because those distros lacked a recent enough binaryen for wasm_of_ocaml) is now obsolete — the opam-repo CI no longer builds wasm_of_ocaml for eliom — and is removed (which empties and so deletes eliom.opam.template).

Verified: with wasm_of_ocaml-compiler removed from the switch, dune build -p eliom still succeeds.

2. Distillery templates: WASM opt-in on the toolchain

A generated project must not require the wasm toolchain unless wasm is actually wanted:

• Makefile.options: ENABLE_WASM auto-detects wasm_of_ocaml (command -v), defaulting to yes when present and no otherwise (was hardcoded yes).
• app.{exe,lib}/dune: drop client/<project>.bc.wasm.js from the <project> alias deps. That dep made make all compile wasm unconditionally regardless of ENABLE_WASM; the alias is now a plain module check, and the gated js target still builds wasm when enabled. (modes js byte wasm) is kept so the gated build works.
• app.lib/Makefile.app: route byte/opt through the gated js target (app.exe already did).
• Makefile.app: if ENABLE_WASM=yes is forced without wasm_of_ocaml installed, the build fails with an actionable hint instead of a cryptic dune "program not found" error.

Also fix two pre-existing staleness bugs in PROJECT_NAME.opam (both templates) that prevented a generated project from building against current eliom:

• eliom was capped at < 12.0.0, rejecting the current 12.0.1 -> bump to >= 12.0.0 & < 13.0.0.
• ocsigen-ppx-rpc is used by the generated dune files but was not declared -> add it.

3. CI

• The build matrix installs wasm_of_ocaml-compiler explicitly (it is no longer pulled as a dependency) so the eliom library still builds with it present.
• New distillery job: generates a project from each template and builds it. Runs on OCaml 4 because the templates' ocsipersist-sqlite backend pulls lwt_log, which requires OCaml < 5. Two phases:
  • JS only - pins use -n so wasm_of_ocaml-compiler is not installed; asserts no .bc.wasm.js is produced (the opt-out path).
  • wasm enabled - installs binaryen + wasm_of_ocaml-compiler, rebuilds, and asserts .bc.wasm.js is produced. This is the only job that exercises wasm_of_ocaml compilation end to end.

🤖 Generated with Claude Code