README quick-start omits Linux system dependencies, causing guaranteed build failure on fresh clone #3087

git clone https://github.com/spacedriveapp/spacedrive
cd spacedrive
just setup

just setup runs bun install then cargo xtask setup. Two separate
failures occur in sequence:

1. cargo xtask setup itself fails on a fresh clone with:
error: no such command: 'xtask'
This is because the xtask cargo alias is defined inside .cargo/config.toml,
which is gitignored and only gets generated by cargo xtask setup itself —
a bootstrap chicken-and-egg problem. Workaround: cargo run -p xtask -- setup
(unaliased form) works on the first run.

2. Once past that, building the release daemon (part of xtask setup) fails
building openssl-sys, then ffmpeg-sys-next's bindgen step, then linking
iroh-relay, with errors like:
The system library `openssl` required by crate `openssl-sys` was not found.
...
/usr/include/limits.h:124:16: fatal error: 'limits.h' file not found
Unable to generate bindings: ClangDiagnostic(...)
...
rust-lld: error: unable to find library -lz
rust-lld: error: unable to find library -lzstd

Root cause: none of libssl-dev, clang (only the libclang runtime lib was
present, not the actual clang toolchain — this is what causes the bindgen
limits.h error specifically), zlib1g-dev, or libzstd-dev are installed,
because nothing in the followed instructions said to install them.

scripts/setup.sh (documented only in CONTRIBUTING.md, not linked from the
README) does install most of these (libssl-dev, clang, llvm-dev,
libclang-dev, etc. via its apt branch) — but even that script doesn't
install zlib1g-dev or libzstd-dev, so the -lz/-lzstd linker errors
persist even after running it.


### Platform and versions

Environment

- OS: Ubuntu (apt-based), fresh install
- rustc/cargo: 1.97.1
- Followed only the README's documented prerequisites (Rust, Bun, just, Python)


### Stack trace

```shell