native_build: fail fast (with the real cause) when Android needs zig but it's missing

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The failure this fixes

A real mix mob.deploy --native --android failure (gist) ended in:

CMake Error at CMakeLists.txt:78 (add_library):
  Cannot find source file:
    /Users/ben/code/bravautwo_mobile/deps/mob/android/jni/mob_nif.c

That error points at the wrong thing. The real cause is printed ~150 lines earlier and then ignored:

zig not on PATH — skipping build.zig step (CMake will compile sources directly)

On mob 0.7+ the Android JNI layer is compiled by build.zig; the C source the CMake fallback wants (deps/mob/android/jni/mob_nif.c) no longer ships with mob (it's mob_nif.zig now). So once build.zig exists but zig is missing, the build cannot succeed — but zig_build_android_objects/4 warned in yellow and returned :ok, limping into Gradle to die later with a CMake error that blames a missing file instead of the missing toolchain.

Change

Extract the decision into a pure zig_build_plan/3 and add a fourth outcome:

build.zig	zig	legacy mob_nif.c	plan	behaviour
no	–	–	:skip_no_build_zig	nothing to do
yes	yes	–	:run_zig	real build.zig path
yes	no	yes	:legacy_cmake	warn + continue (old mob still has the C sources)
yes	no	no	:zig_required	abort before Gradle with an actionable message

The :zig_required path returns {:error, zig_required_message()}, so the build stops immediately and the last thing printed is the cause and the fix:

✗ Android native build failed: zig is not on your PATH, and this project's
Android native build needs it.

mob 0.7+ compiles the Android JNI layer with build.zig. The legacy CMake
fallback would reference C sources (deps/mob/android/jni/mob_nif.c) that no
longer ship with mob, so the build cannot succeed without zig.

Install zig 0.15.x, then re-run `mix mob.deploy --native --android`:
  asdf:    asdf plugin add zig && asdf install zig 0.15.2 && asdf global zig 0.15.2
  manual:  https://ziglang.org/download/  (then put `zig` on your PATH)

Verify your toolchain any time with `mix mob.doctor`.

The version (0.15.x / 0.15.2) matches what mix mob.doctor already recommends. Projects on an older mob that still ships the C sources are unaffected: they keep the :legacy_cmake warn-and-continue path.

Tests

zig_build_plan/3 is tested across the full 2×2×... matrix, and zig_required_message/0 is pinned to name both the cause (mob_nif.c, "zig is not on your PATH") and the fix (zig 0.15, mix mob.doctor). Full native_build_test.exs green (148). format / credo --strict / compile --warnings-as-errors clean (pre-push hook passed).

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