Kani Metrics: aarch64 SVE crash, fail-fast masking, and memory #715

Follow-up from #687 (nightly-2026-09-25), deliberately left out of that PR. Kani Metrics isn't a required check.

• macOS: Kani crashes compiling core on aarch64 SVE scalable-vector tuple types (svint32x4_t, svint64x2_t, …): typ.rs:777 field/offset count mismatch. Tracked as ICE on aarch64 SVE scalable-vector tuple types (svint32x4_t): field/offset count mismatch in codegen_struct_fields kani#4963, fixed by Fix codegen crashes on SVE tuples and non-power-of-two Simd on aarch64 kani#4964 (open). Once it merges, bump tool_config/kani-version.toml to Kani main, provided main is still on our toolchain.
• ubuntu: gets cancelled by the matrix's fail-fast: true as soon as macOS fails, which hides its result. Consider fail-fast: false.
• Memory: Merge subtree update for toolchain nightly-2026-09-25 #687 set CARGO_BUILD_JOBS=1 for Metrics and Kani List. Compiled in parallel, the library crates hit 16 GB under Kani, the runner's total; serially the peak is ~12.6 GB (std). This is slower, so it's worth revisiting if the jobs get too long.