Documented `bazel test //...` workflow cannot run: repository has no Bazel build files

## Description

The [README quick start](https://github.com/google-deepmind/speech-compass#quick-start) and [`dsp/README.md`](https://github.com/google-deepmind/speech-compass/blob/main/dsp/README.md#building-and-testing) document building and testing the DSP algorithms with:

```bash
bazel test //...
bazel test //test:gcc_phat_test
```

However, the repository contains no `MODULE.bazel`, `WORKSPACE`, or `BUILD` files, so every documented command fails immediately:

```
ERROR: The 'test' command is only supported from within a workspace
(below a directory having a MODULE.bazel file).
```

It looks like the Bazel package files were dropped during the export to GitHub — `defs.bzl` still references internal-only labels that cannot resolve in the open-source tree:

- `load("bazel_rules/rules_cc/cc:cc_binary.bzl", ...)` (open-source path is `@rules_cc//cc:...`)
- `CONDITION_WINDOWS = "//tools/cc_target_os:windows"` (open-source label is `@platforms//os:windows`)

Additionally, `test/gcc_phat_test.c` includes `audio_to_tactile/src/dsp/complex.h` from [google/audio-to-tactile](https://github.com/google/audio-to-tactile), which needs to be declared as an external dependency.

## Steps to reproduce

```bash
git clone https://github.com/google-deepmind/speech-compass
cd speech-compass
bazel test //...
```

## Expected

The four tests under `test/` build and pass, as documented.

I have a working fix (MODULE.bazel + BUILD files + `defs.bzl` load-path corrections; `bazel test //...` passes 4/4 on Windows/MSVC 2022 with Bazel 9.1.1, and the tests also pass compiled manually with GCC `-std=c11`) and will send a PR shortly.

Start with the documented commands in the README quick start and dsp/README.md, then inspect defs.bzl and test/gcc_phat_test.c. Run bazel test //... from a fresh checkout and verify that the four tests build and pass on the supported toolchains.

Written by the indexing model from the issue text.