bug: ebuild build ignores ninja_command() and still requires the pip ninja module

[agayushh]

What happened?

ninja_command() in ebuild/build/dispatch.py prefers a ninja binary on PATH and falls back to python -m ninja.

ebuild test already uses that helper (ebuild/cli/commands.py, _run_native_tests).

ebuild build still hardcodes:

ninja_cmd = [sys.executable, "-m", "ninja", "-f", str(build_path / "build.ninja")]

### Expected behavior

```text
`ebuild build` should call `ninja_command()` the same way `ebuild test` does: use `ninja` on PATH when present, and fall back to `python -m ninja` only when no binary is found.

README.md and demo.md should describe that behavior.

### Reproduction steps

1. Open `ebuild/cli/commands.py` at the `ebuild build` ninja invocation (around line 1032).
2. Observe it always builds argv as `[sys.executable, "-m", "ninja", "-f", ...]`.
3. Compare with `_run_native_tests` in the same file (around line 2713), which already uses `ninja_command()`.
4. Compare with `ninja_command()` in `ebuild/build/dispatch.py`, whose docstring says the unconditional `python -m ninja` path was the old bug.
5. Optional: on a machine with `ninja` on PATH but the PyPI `ninja` module not installed in the venv, run `ebuild build` in `examples/hello_world` and `ebuild test` on a project with `type: test` targets. Build fails; test does not.

### Version or commit

c37e995fea8adf096d8babb2592e29967ae89462

### Environment

Linux. This is independent of board/toolchain: it is the argv `ebuild build` passes to ninja, not a compile failure.

### Logs or diagnostics

```shell
Code path on current master (`c37e995`):

ebuild/cli/commands.py:1032
    ninja_cmd = [sys.executable, "-m", "ninja", "-f", str(build_path / "build.ninja")]

ebuild/cli/commands.py:2713
    from ebuild.build.dispatch import ninja_command
    argv = ninja_command() + ["-f", str(build_path / "build.ninja")] + ...

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