Set up sccache as a shared Rust compiler cache during bun install #3087

Problem

Every worktree builds Rust from cold into its own target/ (2.5–23 GB each), and there is no compiler cache shared between them. A new worktree pays a full cold build — minutes of the pre-push gate — on its first push. The same happens on each dev machine.

Solution

Set up sccache as the machine-wide Rust compiler cache as part of bun install (root prepare lifecycle), so every dev machine gets it without a manual step:

• scripts/setup-build-cache.ts, run from the root prepare script
• Install a pinned sccache release into ~/.cargo/bin (SHA-256 pinned in the script) when it is missing
• Configure ~/.cargo/config.toml (build.rustc-wrapper = "sccache", CMake compiler launchers for llama.cpp) — merge only when safe, otherwise print the lines to add
• Set a 40 GiB local disk cache
• Near-zero-cost no-op when already configured; never fails bun install (offline, unsupported platform, no cargo); opt-out via NODESPACE_SKIP_BUILD_CACHE=1

Local cache per machine, not a shared remote cache: cross-machine hits need identical rustc and absolute paths, and the per-machine win (many worktrees sharing one cache) captures most of the benefit.

Acceptance criteria

• bun install on a machine without sccache installs and configures it
• bun install on a configured machine is a fast no-op (no network)
• An existing ~/.cargo/config.toml is never clobbered
• Failure to set up never fails bun install
• Unit tests (DOM-free, under scripts/) for the config-merge decisions