The pinned librustzcash / transitive crates (notably traitobject 0.1.0) rely on
pre-1.70 trait coherence and fail to compile on newer rustc (E0119), so the backend
must build with 1.63. Add a rust-toolchain.toml in the vendored backend so rustup
auto-selects 1.63 when cargo runs there — no more manual RUSTUP_TOOLCHAIN=1.63.0.
The pin is scoped to the backend tree (repo-root cargo keeps the default toolchain).
Also symlink the pin into the prepared build root so --silentdragonxlitelib-dir
builds honor it too.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
DanS
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