--all-features does not build: the garb feature needs symbols that only exist on garb's feat/brag-format branch

@lilith is already working on this.

Since Aug 29, 2026.

## What

`cargo test --workspace --all-features` fails to compile with 8 errors:

```
error[E0425]: cannot find function `rgba_to_brag` in module `garb::bytes`
error[E0425]: cannot find function `brag_to_rgba` in module `garb::bytes`
error[E0425]: cannot find function `bgra_to_brag` in module `garb::bytes`
error[E0425]: cannot find function `brag_to_bgra` in module `garb::bytes`
error[E0425]: cannot find function `rgba_to_brag_inplace` in module `garb::bytes`
error[E0425]: cannot find function `brag_to_rgba_inplace` in module `garb::bytes`
error[E0425]: cannot find function `bgra_to_brag_inplace` in module `garb::bytes`
error[E0425]: cannot find function `brag_to_bgra_inplace` in module `garb::bytes`
error: could not compile `brag` (lib) due to 8 previous errors
```

Default features are fine — `cargo test --workspace` is 6 suites / 48 passed / 0 failed. Only the `garb` feature is affected.

## Why

`Cargo.toml` has:

```toml
garb = { optional = true, git = "https://github.com/imazen/garb" }
```

No `branch` or `rev`, so this tracks garb's **default branch** (`main`). Those eight functions exist only on garb's **`feat/brag-format`** branch — the sole commit defining them is `c437c7ae` (`src/bytes/mod.rs`, `src/bytes/avx2.rs`, `src/bytes/neon.rs`). They have never been on `main`.

The currently locked garb rev, `67c4efb`, is an ancestor of garb `main` and does not define them either, so this is not lockfile drift — the feature has never been buildable against garb `main`.

## Why nobody noticed

This repo has **no `.github/workflows/` at all** — no CI runs on it, so neither the `--all-features` build failure nor anything else is gated.

## Options

1. Point the dependency at the branch that has the symbols: `garb = { optional = true, git = "https://github.com/imazen/garb", branch = "feat/brag-format" }` — best pinned to a `rev` so it cannot drift.
2. Land the BRAG swizzle family on garb `main` and keep tracking the default branch.
3. Drop the `garb` feature if it is superseded by the in-tree `swizzle` feature.

Whichever way it goes, adding a minimal CI workflow that runs `cargo test --workspace --all-features` would stop this from silently rotting again.

## Also noticed (minor, not fixed here)

`cargo fmt --all --check` reports drift in two files that predate this report: `brag-art/benches/kernel_tiers.rs` and `brag-art/src/neon.rs`. Left alone since there is no fmt gate to unblock.