Standardize `<DEP>_BRANCH` override across libneo consumers

[krystophny]

The reverse-dependency validation in the proposed development model
(docs/development-model.md, PR #54) rests on one convention: every consumer's
build honors <DEP>_BRANCH (environment or CMake cache variable). When set,
find_or_fetch fetches that dependency at the given ref instead of the
committed default; unset uses the committed pin; a bad ref fails loudly.

LIBNEO_BRANCH is the libneo instance, and SIMPLE already implements it. Bring
the rest to the same behavior:

• SIMPLE
• NEO-2
• NEO-RT
• MEPHIT
• KAMEL
• rabe

Generalize find_or_fetch to read ${DEP_UPPER}_BRANCH, so the same hook
covers fortplot and any future hub with no new mechanism.

Acceptance: dispatching each repo's build with LIBNEO_BRANCH=<ref> builds
against that libneo, with no commit to the repo. This is the single enabling
piece for the release-time gate (libneo issue) and the per-repo integration.yml.

[krystophny]

LIBNEO_BRANCH override PRs opened for all libneo dependants (additive; unset = unchanged, CMake-parse checked):

• NEO-RT: Unify libneo resolver via LIBNEO_REF, drop $CODE shortcut NEO-RT#41
• MEPHIT: Support LIBNEO_BRANCH override for the libneo dependency MEPHIT#19
• KAMEL: Support LIBNEO_BRANCH override for the libneo dependency KAMEL#132
• NEO-2: Resolve libneo via LIBNEO_REF override, default to release branch NEO-2#86
• rabe: Support LIBNEO_BRANCH override for the libneo dependency rabe#83
• tiago: Support LIBNEO_BRANCH override for the libneo dependency tiago#2

SIMPLE already had LIBNEO_BRANCH. Once these merge, all dependants honor it uniformly, which makes the 26.2 release pins one-liners and enables the release-time dispatch validation. The four find_or_fetch versions also generalize to <DEP>_BRANCH (e.g. fortplot).