fix(zephyr): uppercase root Kconfig symbol ARBITER

All tests and samples use CONFIG_ARBITER=y, but the root Kconfig symbol was
lowercase 'arbiter'. Zephyr Kconfig treated CONFIG_ARBITER as undefined and
aborted Twister. Rename menuconfig/if/endif to ARBITER.

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

Author: tbitcs

LEDGER.md

@@ -1,5 +1,16 @@
 # Change Ledger
 
+## 2026-05-31T23:28 — Fix Zephyr Kconfig root symbol
+- **Author**: Oz
+- **Type**: fix
+- **REQs affected**: REQ-BUILD-001, REQ-ARCH-016
+- **Summary**: Changed the root module symbol in `subsys/arbiter/Kconfig`
+  from lowercase `menuconfig arbiter` / `if arbiter` to uppercase
+  `menuconfig ARBITER` / `if ARBITER`, matching all `CONFIG_ARBITER=y`
+  test/sample configurations and Zephyr Kconfig conventions. This fixes
+  Twister build failures where `CONFIG_ARBITER` was treated as undefined.
+- **Status**: complete
+
 ## 2026-05-31T22:36 — Benchmarks: build_only → execute in CI
 - **Author**: Oz
 - **Type**: ci

subsys/arbiter/Kconfig

@@ -1,14 +1,14 @@
 # SPDX-License-Identifier: MIT
 
-menuconfig arbiter
+menuconfig ARBITER
 	bool "arbiter deterministic reasoning engine"
 	help
 	  Enable the arbiter deterministic reasoning and safety-policy
 	  engine. arbiter evaluates compiled ARB models against frozen
 	  input snapshots to produce deterministic results and
 	  explanation traces.
 
-if arbiter
+if ARBITER
 
 config ARBITER_LOG_LEVEL
 	int "arbiter log level"
@@ -136,4 +136,4 @@ config ARBITER_WATCHDOG
 	  On evaluation failure or timeout, the watchdog is not fed,
 	  allowing it to trigger a reset.
 
-endif # arbiter
+endif # ARBITER