Zed's `fs_embed!` macro hand-implements rust-embed's `RustEmbed` trait
in its dev arm so debug builds read assets from the checkout at runtime
(introduced with corgi support in #63396). That trait returns
`rust_embed::Filenames`, an enum with exactly one variant per
compilation context: `Dynamic` when rust-embed's `debug-embed` feature
is off, `Embedded` when it is on. The dev arm constructed
`Filenames::Dynamic` unconditionally.

Cargo unifies features across the entire build graph, so a downstream
workspace that consumes Zed's crates by git dependency and enables
`rust-embed/debug-embed` anywhere (Delta did, for self-contained debug
binaries) changes the enum's shape for `util` itself, and `util` stops
compiling with `E0599: no variant named Dynamic`. Zed's own CI can never
catch this because nothing in Zed's graph enables the feature; it only
surfaces in embedding workspaces, where it blocks any dependency bump
past #63396.

This change makes the combination supported. `util` gains a forwarding
`debug-embed` feature, and `__fs_embed_iter` now has two implementations
under cfgs that mirror rust-embed's variant availability: the existing
boxed-iterator path when the feature is off, and an `Embedded` path when
it is on, which scans the checkout once per embed site and leaks the
cached name list (the `Embedded` variant requires `'static` names; a
per-process list matches the macro's documented contract that dev edits
appear on the next launch, and `get` still reads file contents fresh on
every call). Consumers must enable the feature through
`util/debug-embed` rather than directly on rust-embed — enabling it
behind util's back still breaks, which only rust-embed itself could fix;
the feature's documentation says so.

A new test exercises the dev arm's `iter` and `get` through the macro,
and running util's suite with `--features debug-embed` covers the other
variant; both configurations pass 133 tests, plus clippy and fmt.

Release Notes:

- N/A

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>