Extend cast lowering coverage

[uurl]

Part of #303.

Summary

Adds additional focused unit coverage for existing MIR cast lowering paths.

The existing cast-lowering tests covered representative cases for integer widening/narrowing, unsigned integer-to-float, signed float-to-int saturation, and pointer address exposure. This PR extends that coverage to additional documented cast dispatch paths in cast.rs.

What changed

Adds tests for:

• Same-width integer casts lowering to llvm.bitcast.
• Signed integer-to-float casts lowering to llvm.sitofp.
• Unsigned float-to-integer casts lowering through the expected llvm_fptoui_sat_* intrinsic.
• Float widening casts lowering to llvm.fpext.
• Float narrowing casts lowering to llvm.fptrunc.
• Pointer-with-exposed-provenance casts lowering to llvm.inttoptr.
• Pointer-to-pointer casts across address spaces lowering to llvm.addrspacecast, including verification of the resulting address space.

Testing

• cargo fmt --all -- --check
• cargo test -p mir-lower convert::ops::cast::tests --lib
• cargo test -p mir-lower
• cargo clippy --workspace -- -D warnings

Checklist

• Test coverage only; no compiler behavior change
• No crates/cuda-bindings/ changes