Support LIBNEO_BRANCH override for the libneo dependency

[krystophny]

Wires MEPHIT into libneo's per-PR reverse-dependency gate (full=yes downstream) and makes the build hermetic with respect to $CODE.

Dispatch contract

Implements the gate's dispatch contract for MEPHIT's main.yml:

• main.yml workflow_dispatch inputs: libneo_ref (the libneo ref to build against) and full (accepted for uniform dispatch). LIBNEO_BRANCH is set from inputs.libneo_ref.
• cmake/Util.cmake: find_or_fetch honors a <DEP>_BRANCH env override. For libneo that is LIBNEO_BRANCH, so a gate dispatch with -f libneo_ref=<sha> builds MEPHIT against that candidate libneo via FetchContent (GIT_TAG ${REMOTE_BRANCH}). Unset keeps the current branch/main resolution. Additive.

MEPHIT CI is build-only (no ctest/pytest tests are registered with the build), so building against the candidate libneo is itself the gate; full=true changes nothing and is accepted only so the gate can dispatch every full=yes downstream uniformly.

Normal push/PR builds are unchanged.

Hermetic build (no $ENV{CODE})

Remove every $ENV{CODE} path from the CMake build so a clean checkout configures without an ambient $CODE tree. libneo is now built from source by default.

• cmake/Util.cmake find_or_fetch: dropped the $ENV{CODE}/<dep> source bypass. Dependencies default to FetchContent; -D<DEP>_PATH (e.g. -DLIBNEO_PATH=<dir>) remains the explicit local-source fast override. The function now also exports <DEP>_DIR as the dependency build dir.
• cmake/SetupCODE.cmake: dropped the if(DEFINED ENV{CODE}) block that pre-seeded LIBNEO_DIR, FGSL_DIR, TRIANGLE_DIR, MFEM_DIR from $CODE. LIBNEO_DIR was unused by the build (build links the LIBNEO:: FetchContent targets). FGSL_DIR is unused (build uses find_package(GSL) + GSL::gsl). Triangle and MFEM keep their -DTRIANGLE_DIR= / -DMFEM_DIR= cache options and the FetchContent/ExternalProject fallback in SetupTriangle.cmake / SetupMFEM.cmake.
• Runtime binary: libneo's FetchContent subbuild builds vacfield.x (via libneo CMakeSources.in) into the dependency build dir. find_or_fetch exports that as LIBNEO_DIR, so vacfield.x is at $MEPHIT_DIR/libneo/vacfield.x instead of $CODE/libneo/build/vacfield.x. README updated accordingly.

Consequence: MEPHIT builds libneo from source by default (hermetic, slower). -DLIBNEO_PATH=<dir> is the local-dev fast path.

Verification

rg -i '\$ENV\{CODE\}' over cmake/, CMakeLists.txt, Makefile returns nothing.

Configure with CODE set in env still fetches libneo from git, not from $CODE:

$ CODE=/home/ert/code cmake -S . -B /tmp/mephit-cfg -G Ninja
...
-- Using libneo ref main from https://github.com/itpplasma/libneo.git
...
-- Configuring done
-- Generating done

LIBNEO_DIR cache resolves to the fetched build dir (<build>/libneo), and vacfield.x is a build target (ninja -t targets | grep vacfield -> vacfield.x).

Tracking itpplasma/code#55, gated by itpplasma/libneo#278.

[krystophny]

@phizenz heads-up: this PR is part of a cross-repo change unifying how the codes resolve libneo (a validated LIBNEO_REF override and tracking the release/26.2 branch by default), plus an external CI trigger so the libneo release gate can build this code against a candidate ref. It needed a few more changes than expected, so your feedback would be valuable before it lands. We plan to merge the whole set on Monday, 2026-06-15. Companion CI PR: #23.