feat: implement 31 missing tensor ZRTL operations

Add all tensor operations referenced by grammar builtins but missing
from the ZRTL tensor plugin, eliminating all "can't resolve symbol"
runtime panics:

- Element-wise math: abs, sqrt, exp, log, sin, cos, tanh, pow, clamp
- Activations: relu, sigmoid, softmax
- Comparisons: eq, ne, lt, le, gt, ge (return boolean f32 tensors)
- Axis reductions: sum_axis, mean_axis, max_axis, min_axis
- Statistics: std (standard deviation), var (variance)
- Shape ops: flatten, transpose_axes
- Conversion: to_list, item, cross_entropy
- Stubs for list-based ops: concat, stack, split

Uses helper functions (unary_elementwise_f32, binary_cmp_f32,
axis_reduce_f32) to avoid repetitive iteration boilerplate.

Author: darmie