zephyr: initial integration

[rootvector2]

Initial OSS-Fuzz integration for the Zephyr RTOS
(https://github.com/zephyrproject-rtos/zephyr).

Build path: native_sim/native/64 with CONFIG_ARCH_POSIX_LIBFUZZER=y,
which is Zephyr's upstream-supported libFuzzer entry point. No patches
to Zephyr are required.

• Dockerfile: pulls a Zephyr workspace via west init + west update,
  and builds Python 3.12 from source (Zephyr's CMake requires >= 3.12,
  base-builder ships 3.11).
• build.sh: maps OSS-Fuzz's $SANITIZER onto Zephyr's CONFIG_ASAN,
  CONFIG_UBSAN, CONFIG_MSAN Kconfigs and drives
  west build -b native_sim/native/64 samples/subsys/debug/fuzz.
• One initial fuzz target (zephyr_fuzz_sample, the in-tree fuzz
  harness). More targets can be added by extending build.sh.

Tested locally with Docker:

• build_fuzzers succeeds for SANITIZER=address and SANITIZER=undefined.
• check_build (test_all.py) passes for both sanitizers.
• run_fuzzer exercises the binary at ~22k exec/s and grows coverage as
  expected (cov 211 / ft 222 in 16s).

The Zephyr security team has been notified upstream and the primary
contact in project.yaml will be confirmed there before merge.

[github-actions]

rootvector2 is integrating a new project:
- Main repo: https://github.com/zephyrproject-rtos/zephyr.git
- Criticality score: 0.75463

[DavidKorczynski]

project itself looks good, but we need maintainer coordination and approval.