DAG DSL → StableHLO: multi-input concatenate mis-infers the concatenated axis (0.27.0)

Surfaced by the conformance harness. A 3-input concat along dim=1 infers the output
shape with the channel extent of the first operand instead of the sum of all operands.
The 2-input concat in the op micro-suite produces the correct shape, so this is
arity/axis-specific — distinct from #663 (self-containment) and #666 (reshape/norms).

Repro (verified, 0.27.0)

A YOLO-style detection head concatenating three branches (1,1,8,8) + (1,4,8,8) + (1,1,8,8)
along the channel axis emits:

%out = stablehlo.concatenate %a, %b, %c, dim = 1
     : (tensor<1x1x8x8xf32>, tensor<1x4x8x8xf32>, tensor<1x1x8x8xf32>)
    -> tensor<1x1x8x8xf32>     // EXPECTED: tensor<1x6x8x8xf32>

The result type keeps 1 on the concatenated axis (operand 0's extent) instead of
1+4+1 = 6. iree-compile then rejects the module (operand/result shapes inconsistent).

Expected

concatenate over dim = d should set the result extent on d to the sum of the operands'
extents on d (here 6), for any number of operands.

cc #663