Zed's `fs_embed!` macro normally reads assets from the checkout in debug
builds. The new `util/debug-embed` feature is intended to compile those
assets into debug binaries, but it previously changed only the
`rust_embed::Filenames` variant while leaving file loading on the
runtime path.

That breaks downstream workspaces such as Delta: the runtime loader
identifies Delta as the repository root, then looks for Zed grammar
assets under Delta's checkout. Tests consequently panic when language
configuration files cannot be found.

This makes `util/debug-embed` select a compile-time `RustEmbed`
expansion in every profile. Builds without the feature retain live
filesystem loading in debug mode. The runtime iterator no longer depends
on a feature-specific `rust_embed::Filenames` variant, so Cargo feature
unification remains safe when another package enables
`rust-embed/debug-embed` directly. The regression fixture uses a
deliberately invalid runtime path when `debug-embed` is enabled, so it
proves that the embedded path is used rather than passing only because
the Zed checkout contains the test assets.

Testing performed:

- `cargo fmt --all -- --check`
- `cargo nextest run -p util --lib`
- `cargo nextest run -p util --lib --features debug-embed`
- `cargo check -p remote_server --features debug-embed`
- `./script/clippy -p util`
- Delta's previously failing
`tools::edit_file::tests::test_edit_file_rejects_scratch_paths_as_read_only`
against this PR's head commit

Release Notes:

- N/A