fix(zephyr): update module CMake guard from CONFIG_ZPROJ to CONFIG_ARBITER

The module CMakeLists still guarded library/include registration on legacy
CONFIG_ZPROJ, so enabling CONFIG_ARBITER did not add include/ or runtime
sources. Switch guards to CONFIG_ARBITER.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: tbitcs

CMakeLists.txt

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: MIT
 
-if(CONFIG_ZPROJ)
+if(CONFIG_ARBITER)
   zephyr_library()
   zephyr_library_sources(
     lib/ARBITER_engine.c
@@ -19,5 +19,5 @@ if(CONFIG_ZPROJ)
 
   zephyr_include_directories(include)
 
-  add_subdirectory_ifdef(CONFIG_ZPROJ subsys/arbiter)
+  add_subdirectory_ifdef(CONFIG_ARBITER subsys/arbiter)
 endif()

LEDGER.md

@@ -1,5 +1,15 @@
 # Change Ledger
 
+## 2026-05-31T23:39 — Fix Zephyr module CMake guard
+- **Author**: Oz
+- **Type**: fix
+- **REQs affected**: REQ-BUILD-001, REQ-ARCH-016
+- **Summary**: Updated root `CMakeLists.txt` guards from legacy
+  `CONFIG_ZPROJ` to `CONFIG_ARBITER`. This ensures the module library,
+  `include/` path, runtime sources, and `subsys/arbiter` integration are
+  actually registered when tests and samples set `CONFIG_ARBITER=y`.
+- **Status**: complete
+
 ## 2026-05-31T23:28 — Fix Zephyr Kconfig root symbol
 - **Author**: Oz
 - **Type**: fix