feat(install): one git install command, with the studio behind a studio feature

[LeadcodeDev]

cargo install --git https://github.com/LeadcodeDev/rustmotion                     # CLI
cargo install --git https://github.com/LeadcodeDev/rustmotion --features studio   # CLI + studio

Both verified by running them, not by reasoning about them — see Proof below.

Why this needed more than a manifest line

The bare command did not work at all before:

error: multiple packages with binaries found: rustmotion, rustmotion-studio.
When installing a git repository, cargo will always search the entire repo for
any Cargo.toml.

That check is not about the workspace. Cargo scans the whole repository and
demands that exactly one package declare a [[bin]]. Two candidate escapes
were tested and both failed:

Attempt	Result
default-members = ["crates/rustmotion", "crates/rustmotion-studio"]	same error
required-features on rustmotion's bin, to hide it from the scan	same error

So one package owns both binaries, and it has to be rustmotion: cargo install rustmotion from crates.io must keep delivering a command, which a crate reduced
to a [lib] does not.

That was blocked by a cycle. rustmotion-studio depended on rustmotion for
loader and encode, so making it a dependency of rustmotion is a cycle —
and cargo refuses one even for an optional dependency, which the old manifest
comment already said.

What it does instead

The studio becomes a module: crates/rustmotion/src/studio/. With no second
package there is no cycle. Because lib.rs already declares extern crate self as rustmotion, every one of the studio's rustmotion::loader::… paths stays
valid verbatim — the only code change is requalifying its 82 internal crate::
paths to crate::studio::, plus rustfmt rewrapping the longer lines.

The alternative was extracting loader, encode, include, assets and the
engine extensions into a third crate that both sides depend on. It reaches the
same command, moves 14 900 lines instead of 11 600 unchanged ones, and adds two
crates to the published set. I started down it and backed out.

studio is out of the default build deliberately: it pulls gpui and a native
GUI toolchain, which do not build everywhere the CLI builds — a headless server
being the obvious case. Its seven dependencies are optional, and the second
[[bin]] carries required-features = ["studio"].

Two consequences worth naming

CI would have gone quietly blind. --workspace compiled the studio because
it was a member. Behind a non-default feature it does not, so clippy and the
tests would have stayed green over 11 600 lines nobody checks any more. Both
jobs now pass --features rustmotion/studio. Test count is unchanged:

Build	Tests
default	1510
--features rustmotion/studio	1701 — the same total as before the merge

serde_json/preserve_order moves into the studio feature, not the
default. The studio rewrites the user's scenario files and must not reorder
their keys. validate --fix would want the same guarantee, but enabling it by
default changes the key order of every file it rewrites, which is a separate
decision and not this PR's.

Proof

$ cargo install --git file://$PWD --locked
   Installed package `rustmotion v0.7.1` (executable `rustmotion`)

$ cargo install --git file://$PWD --features studio --locked
   Installed package `rustmotion v0.7.1` (executables `rustmotion`, `rustmotion-studio`)

Both installed binaries then run: rustmotion --help lists all twelve
subcommands with their descriptions intact, rustmotion-studio --help prints
its own, and the feature-less CLI validates a scenario.

cargo fmt --all --check, cargo clippy --workspace --all-targets --features rustmotion/studio -- -D warnings and both test configurations are clean.

Also in here

• publish = false is gone with the package. The studio's git dependency that
  blocked publishing is long gone — Cargo.lock has zero git+ sources and
  both gpui crates resolve from the registry — so the flag was stale.
• docs/studio-dioxus-notes.md: the studio's DIOXUS_NOTES.md had nowhere left
  to live. It documents a framework the studio no longer uses; moved rather than
  deleted, since that is not this PR's call.
• CLAUDE.md's "no comments" rule follows the folder now instead of the crate.
  The reasoning did not change with the merge.
• The cli/mod.rs note claiming a studio subcommand is impossible was
  rewritten: it is possible now and still unwanted, because it would have to
  vanish from --help depending on the feature.