fix(release): name the server assets for whoever downloads them, not for cargo (#265)

`tty7-server-x86_64-unknown-linux-musl` was never a name anyone chose. Both
workflows staged the file as `tty7-server-${{ matrix.target }}`, so the build
triple went straight into a published filename — and the triple's *vendor*
field, for a Linux target with no particular vendor, is the literal word
`unknown`. It has been sitting on the releases page reading like a failed
lookup.

Of the triple's four fields only two say anything to whoever downloads this:
the architecture, which is what `asset_for_uname` picks by, and `musl`, which
is why one file runs on any distribution. So:

    tty7-server-x86_64-unknown-linux-musl  →  tty7-server-linux-x86_64-musl
    tty7-server-aarch64-unknown-linux-musl →  tty7-server-linux-aarch64-musl

`<os>-<arch>` in that order because that is what the GUI assets in the same
release already use (`tty7-<version>-linux-x86_64.tar.gz`). One release should
be one naming scheme; it was two.

The triple stays everywhere it really is a build target — `cargo zigbuild
--target`, the `target/<triple>/release` path, the rust-cache key, ci.yml's
matrix. The workflows now carry both: `target` for the build, `asset` for the
filename, deliberately not the same string.

This name is a contract with more than the release step, and all of it moves
together:

- `install::asset::{ASSET_X86_64, ASSET_AARCH64}`, which is what the client
  appends to a release URL.
- `bundle-windows.ps1`, which stages the musl binary for WSL. `wsl.rs` looks
  for `<dir>/<asset name>` with nothing translating, so the *filename* is as
  much a contract as the `server/` directory is — now said out loud in both
  places, along with the consequence for `TTY7_BUNDLED_SERVER_DIR`: a
  cross-compile has to be copied to the asset name, not left as `tty7-server`.
- The GUI's install prompt fixture, the checksum manifest fixtures, and the
  `MissingBundled` assertions.

Nothing globs the old shape: `gh release upload dist/*`, `checksums.txt`'s
`find`, and the installer's `server\*` are all name-agnostic.

A new test pins both names as literals — the module header already says this
naming is "a *literal* contract with the release workflow", and asserting the
consts against themselves asserted nothing. It also fails on the substring
`unknown`, since that word only ever arrived here by way of `matrix.target`,
and checks neither name contains the other, which is what
`checksums::expected_digest` says out loud that it relies on.

Compatibility: a stable client asks its own frozen tag, which keeps whichever
name it shipped with, so every released client keeps working. The rolling
`nightly` tag is replaced each night and its prune step drops assets the run
did not upload — so an *already installed* nightly client 404s on the server
download until it updates itself. Accepted deliberately; the next release is
what has to be right.

Co-authored-by: thomas <thomas@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/scripts/bundle-windows.ps1

@@ -33,7 +33,11 @@ Copy-Item README.md "$Stage/README.md"
 # change on its own. Missing is a warning, not an error, matching `server-musl`'s
 # own skip-don't-fail probe; WSL then fails at connect time with a message
 # naming the directories it searched.
-$ServerAsset = "tty7-server-x86_64-unknown-linux-musl"
+#
+# The *filename* is a contract too, not just the directory: `wsl.rs` looks for
+# `<dir>/<asset name>`, so this string has to stay whatever
+# `install::asset::ASSET_X86_64` says it is.
+$ServerAsset = "tty7-server-linux-x86_64-musl"
 $ServerSrc = "bundled-server/$ServerAsset"
 if (Test-Path $ServerSrc) {
     New-Item -ItemType Directory -Force -Path "$Stage/server" | Out-Null