Add boundary modes to `@stencil`

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What this task asks for

Add configurable boundary handling modes to `@stencil` for out-of-bounds accesses. Default `cval` is 0. Use `@stencil('wrap')` for a single mode, or `mode=('wrap', 'nearest')` for per-dimension control. For reflect and symmetric modes, if the reflected index is still out of bounds, use `cval` for that access. Invalid mode raises `NumbaValueError`. Mode tuple length must match array dimensions. The `mode` parameter must work alongside existing stencil options: `cval`, `neighborhood`, and `standard_indexing`.