fix(ci): re-pin rust-ci-reusable past toolchain-input breakage (#76)

Re-pins `rust-ci-reusable.yml` from
`d135b05bfc647d0c0fbfedc7e80f37ea50f49236` (pre-fix) to
`412a7031577112b31ee287cc6060179d638d6500` (standards main).

**Why:** the old SHA predates standards#439. Under a SHA-pinned
`dtolnay/rust-toolchain`, the `toolchain` input is mandatory; without it
all four rust-ci jobs (`check`/`test`/`audit`/`coverage`) fail
immediately with `'toolchain' is a required input` — an estate-wide Rust
CI breakage. This PR restores green here.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/rust-ci.yml

@@ -21,4 +21,4 @@ permissions:
 jobs:
   rust-ci:
     timeout-minutes: 30
-    uses: hyperpolymath/standards/.github/workflows/rust-ci-reusable.yml@d135b05bfc647d0c0fbfedc7e80f37ea50f49236
+    uses: hyperpolymath/standards/.github/workflows/rust-ci-reusable.yml@412a7031577112b31ee287cc6060179d638d6500