onnx: add Swish, Mish, Gelu, and RMSNormalization op handlers

Swish (opset 23) maps directly to silu() from core.

Mish (opset 18) expands to x * tanh(ln(1 + exp(x))).

Gelu (opset 20) dispatches on the `approximate` attribute:
- "tanh"  → gelu_approximate(false) from core (cubic tanh approx)
- "none"  → GeluExact expansion: x * 0.5 * (1 + erf(x / sqrt(2)))

RMSNormalization (opset 23) expands to mean-of-squares + rsqrt
normalization over all axes from `axis` onward, followed by scale
multiplication and optional bias addition. Computation is stashed in
F32 for precision, matching the ONNX spec's stash_type default.

Manifest additions enable 52 new tests across test-onnx-core:
- test_mish (1.13.0+)
- test_gelu_default_{1,2}, test_gelu_tanh_{1,2} (1.19.1+)
- test_rms_normalization_* 19 variants (1.19.1+)
- test_swish (1.19.1+)