ci: enforce reproducible Rust toolchain

fixtures/acceptance/coverage.tsv

@@ -12,6 +12,8 @@ S0-ARCH-004	covered	cargo xtask policy scans workspace-owned Rust/TOML for unsaf
 S0-ARCH-005	covered	cargo xtask policy rejects Python source files, Python shebangs, and Python CI workflow steps while allowing docs requirements.txt
 S0-ARCH-006	covered	cargo xtask policy rejects non-fparkan package directories under crates/
 S0-ARCH-007	covered	cargo xtask ci runs fmt, policy, workspace test, clippy, rustdoc warnings, cargo-deny or built-in supply-chain fallback, and strict acceptance audit
+S0-ARCH-008	covered	cargo xtask policy rejects moving Rust toolchains and workspace rust-version drift
+S0-ARCH-009	covered	.github/workflows/ci.yml runs a pinned MSRV backend-neutral crate job
 S0-DIAG-001	covered	cargo test -p fparkan-diagnostics --offline diagnostic_chain_preserves_context
 S0-DIAG-002	covered	cargo test -p fparkan-diagnostics --offline json_is_stable
 S0-CORPUS-001	covered	cargo test -p fparkan-corpus --offline deterministic_traversal_is_creation_order_independent

fixtures/acceptance/stage_0_2_roadmap.md

@@ -12,6 +12,8 @@
 `S0-ARCH-005`
 `S0-ARCH-006`
 `S0-ARCH-007`
+`S0-ARCH-008`
+`S0-ARCH-009`
 `S0-DIAG-001`
 `S0-DIAG-002`
 `S0-CORPUS-001`