[BugFix] Align default (C) tirx.round lowering to ties-to-even

[LngelKyo]

#19368 aligned tir.round to ties-to-even "across all backends" and moved LLVM, CUDA, NVPTX, ROCm, Hexagon, Metal, OpenCL, SPIR-V and WebGPU. It did not touch src/target/intrin_rule.cc, so the default.FLowerIntrinsic rule — which the c target uses — still lowers through FloatSuffix and emits round/roundf, i.e. ties away from zero.

This disagrees with:

• constant folding, which uses std::nearbyint (src/tirx/op/op.cc);
• the CUDA rule, whose own comment says "Use nearbyint (ties-to-even) for round to match constant-folding semantics";
• every other backend after [BugFix] Align tir.round to ties-to-even across all backends #19368.

Why CI did not catch it. test_target_codegen_c_host.py::test_round compiles with target="c" and asserts against np.round, which is ties-to-even — so the intended semantics was already encoded. It passes only because its input is np.random.rand, which does not produce exact midpoints (0 occurrences in 2,048,000 sampled values). The ties-to-even test added by #19368 (test_tir_intrin.py::test_round_ties_to_even) only runs target="llvm", so no test covered the C path's tie behaviour.

The first commit adds the midpoint vector — reused verbatim from test_round_ties_to_even — to the existing C-host test and it fails:

E   Mismatch at indices:
E    [0]: 1.0 (ACTUAL), 0.0 (DESIRED)
E    [2]: 3.0 (ACTUAL), 2.0 (DESIRED)
E    [4]: -1.0 (ACTUAL), -0.0 (DESIRED)
E    [6]: -3.0 (ACTUAL), -2.0 (DESIRED)

The second commit renames round→nearbyint before the float suffix, mirroring the existing CUDA rule, and it passes.

One note for reviewers: nearbyint honours the current floating-point environment and is ties-to-even under the default FE_TONEAREST. That is the same guarantee LLVM, CUDA, Metal and OpenCL already rely on after #19368; this PR does not introduce a new assumption.

[LngelKyo]

@swjng @tlopex — cc, since this completes the backend sweep from #19368.

CI has not been triggered on this PR (no check runs); I believe it needs approval as this is my first contribution here. Happy to rebase or adjust if anything is needed.