fix(ci): re-pin rust-ci-reusable past toolchain-input breakage (#40)

Re-pins rust-ci-reusable.yml from
d135b05bfc647d0c0fbfedc7e80f37ea50f49236 (pre-fix) to
412a7031577112b31ee287cc6060179d638d6500 (standards main). The old SHA
predates standards#439, which added the mandatory `toolchain: stable`
input to the SHA-pinned dtolnay/rust-toolchain step. Without it, all
four rust-ci jobs fail fast with `'toolchain' is a required input`. This
restores Rust CI.

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## Summary

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with "Closes #N". -->

## Changes

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-

## RSR Quality Checklist

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merged. -->

### Required

- [ ] Tests pass (`just test` or equivalent)
- [ ] Code is formatted (`just fmt` or equivalent)
- [ ] Linter is clean (no new warnings or errors)
- [ ] No banned language patterns (no TypeScript, no npm/bun, no
Go/Python)
- [ ] No `unsafe` blocks without `// SAFETY:` comments
- [ ] No banned functions (`believe_me`, `unsafeCoerce`, `Obj.magic`,
`Admitted`, `sorry`)
- [ ] SPDX license headers present on all new/modified source files
- [ ] No secrets, credentials, or `.env` files included

### As Applicable

- [ ] `.machine_readable/6a2/STATE.a2ml` updated (if project state
changed)
- [ ] `.machine_readable/6a2/ECOSYSTEM.a2ml` updated (if integrations
changed)
- [ ] `.machine_readable/6a2/META.a2ml` updated (if architectural
decisions changed)
- [ ] Documentation updated for user-facing changes
- [ ] `TOPOLOGY.md` updated (if architecture changed)
- [ ] `CHANGELOG` or release notes updated
- [ ] New dependencies reviewed for license compatibility (MPL-2.0 /
MPL-2.0)
- [ ] ABI/FFI changes validated (`src/interface/abi/` and
`src/interface/ffi/` consistent)

## Testing

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## Screenshots

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change. -->

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

Author: hyperpolymath