Set up Linux dev environment: build/run fixes + Cursor Cloud docs

[samwdp]

Summary

Sets up the Volt development environment on Linux, gets the SDL3 editor building/running end-to-end, and makes the full cargo xtask ci gate (fmt-check, check, clippy -D warnings, tests) pass on all platforms. Rebased onto latest master. Adds a ## Cursor Cloud specific instructions section to Agents.md.

Code changes (all fix pre-existing cross-platform breakage on master)

Build / runtime

1. editor-syntax — compile_command() selected the MSVC path via runtime if cfg!(windows), compiling both arms on every target and referencing #[cfg(windows)]-only items → Linux/macOS E0425. Now compile-time gated so the MSVC helper, InstallCommandSpec::with_env, and find-msvc-tools are Windows-only.
2. editor-sdl/browser_host.rs — initialize GTK before constructing the wry::WebContext, fixing the Linux startup panic (GTK has not been initialized).

cargo xtask ci clippy (-D warnings)
3. editor-syntax — the gated MSVC branch's trailing return became a needless_return on Windows; return it as a tail expression (verified the cfg-gated block still yields the fn value).
4. editor-sdl — gate the std::env import to Windows (its only user); mark the Windows-only launch_env initial value allow(unused_assignments) off Windows.

cargo xtask ci tests
5. editor-syntax — registry_* tests hard-coded \ separators (not split by Path on Linux); use / (portable, accepted on Windows too).
6. volt-user — buffer_picker_label used Path::file_name (doesn't split \ off Windows); extract the final component treating both / and \ as separators, so Windows-style buffer paths render a clean label on every host.
7. editor-fs (Windows-only test) — compare canonicalized repo roots so the worktree assertion is stable against 8.3 short-name vs long-name expansion (RUNNER~1 vs runneradmin) on the Windows runner.

Environment

• Rust stable (>= 1.91, edition 2024); Linux native libs per .github/workflows/ci.yml.
• Update script: cargo fetch. GUI runs on the VNC desktop (DISPLAY=:1, LIBGL_ALWAYS_SOFTWARE=1).

Verification

• ✅ cargo xtask ci — fully green on Linux (fmt-check, check, clippy -D warnings, all test binaries pass).
• ✅ Windows target cross-checks (via x86_64-pc-windows-gnu + mingw): cargo clippy -p editor-syntax --all-targets --target x86_64-pc-windows-gnu -- -D warnings and the same for editor-fs both pass, confirming the needless_return fix and Windows-gated test compile cleanly. (The Windows-only editor-fs worktree test can't be executed off Windows, but is compile/clippy-checked and the 8.3 normalization is standard.)
• ✅ cargo run -p volt — visible SDL shell + Vim-style text editing (below).

volt_rebased_hello_world_retest.mp4

Volt editor with typed text after rebase

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