Remove all the token structs from core_arch, adjust the generator to emit the wrapper structs directly without core_arch references

Author: Shnatsel

.codex

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+This is a library providing high-level abstractions over SIMD intrinsics in Rust. The library itself is under fearless_simd/, but the files in fearless_simd/generated are auto-generated by the code in fearless_simd_gen/. Do not edit the generated files directly; instead change the generator. You can run the generator with `cargo run -p fearless_simd_gen`.

AGENTS.md

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+# Other Issues
+
+- Out-of-range shift semantics are inconsistent across backends. Some fallback paths use Rust scalar shifts, while SIMD backends use hardware shift instructions; shift counts at or above the lane width can panic or produce backend-specific results.
+
+- On x86_64, Miri currently returns a different NaN sign bit for `f32::NAN.trunc().to_bits()` than native execution. This shows up in `trunc_f32x4_special_values_fallback` and `trunc_f32x8_special_values_fallback`; the reduced repro and issue draft are in `miri_f32_trunc_repro/`. The reduced repro passes on AArch64 under both Miri and qemu-native execution.