[nihalpasham]

Summary

Follow-up cleanup after moving cuda-oxide to stable Rust 1.99.1 (#1609) and landing cute-rs-preview (#1598):

1. cuda-oxide/flake.lock + flake.nix   -> bump rust-overlay for Stable 1.99.1 + set RUSTC_BOOTSTRAP=1
2. cargo oxide new (scaffold.rs)       -> write .cargo/config.toml with RUSTC_BOOTSTRAP = "1"
3. check-toolchain-parity.sh           -> verify flake.lock's rust-overlay rev has the pinned manifest
4. .github/workflows/unit-tests.yml    -> drop unused apt llvm-21 install (sysroot llvm-tools provides LLVM 23)
5. CONTRIBUTING / AGENTS / README / book -> update remaining nightly wording + clean up book backticks & SVG

Changes

• Nix flake (cuda-oxide/flake.lock, cuda-oxide/flake.nix):
  • Bump oxalica/rust-overlay from c1cfa7a2c8 (2026-10-11, two days before 1.99.1 was published) to 5f267b7f32 (2026-10-22) so nix develop finds Stable 1.99.1.
  • Set RUSTC_BOOTSTRAP = "1" in flake.nix (cargoOxideCommonArgs and devShells.default.shellHook).
  • Use a deterministic per-user directory (${XDG_RUNTIME_DIR:-/tmp}/cuda-oxide-nvidia-driver-$(id -u)) for non-NixOS driver symlinks so nix develop --command does not leak /tmp/nix-nvidia-driver.XXXXXX directories.
• Project scaffold (crates/cargo-oxide/src/commands/scaffold.rs):
  • Write .cargo/config.toml ([env]\nRUSTC_BOOTSTRAP = "1"\n) in cargo oxide new so plain cargo check, cargo build, and rust-analyzer work out of the box in standalone projects on stable 1.99.1.
• Toolchain parity guard (scripts/check-toolchain-parity.sh):
  • Verify via an HTTP HEAD check that the oxalica/rust-overlay revision locked in cuda-oxide/flake.lock actually contains manifests/stable/<channel>.nix (or manifests/nightly/...), and document the nix flake update rust-overlay step in check-toolchain-parity.sh and compiler/rustc-public.md.
• CI workflow (.github/workflows/unit-tests.yml, .github/workflows/status-guard.yml):
  • Remove the redundant Install LLVM 21 step on cuda-oxide-codegen (LlvmToolchain::resolve already selects the Rust sysroot's LLVM 23 llc/opt from llvm-tools).
  • Update the status-guard.yml comments to match the current single SIMT stable pin and flake.lock check.
• Contributor, agent, and book docs (CONTRIBUTING.md, AGENTS.md, cuda-oxide/README.md, cuda-oxide-book/):
  • Replace remaining "nightly toolchain" wording in CONTRIBUTING.md, AGENTS.md (CLAUDE.md), cuda-oxide/README.md, getting-started/installation.md, and getting-started/hello-gpu.md.
  • Remove backticks from # sidebar page titles, ## / ### section headings, and prose crate/tool names across the cute/ chapter and compiler/rustc-public.md.
  • Scale bar widths in models/qwen3_8b_cute/svg/03-rtx5090-benchmark-comparison.svg so the right-hand throughput labels fit inside the panel with >=45px margin.

Validation

• cargo test -p cargo-oxide scaffold
• bash cuda-oxide/scripts/check-toolchain-parity.sh
• just fmt-check
• just check-guards
• just book (Sphinx HTML build with -W)

Checklist

• All commits signed off (git commit -s)