gh-216: add intersect

[LuhTonkaYeat]

Adds exec_intersect --- set intersection of two sorted key-value arrays with a
binary operation applied to matched values.

Implementation

• CPU: cpu_intersect.hpp (two-pointer merge).
• OpenCL: cl_intersect.hpp + kernel intersect.cl.
• Public API, schedule task, registry entries (INT/UINT/FLOAT), C binding
  spla_Exec_intersect.

OpenCL notes

• Result stored as a coordinate (COO) vector so genuine zero keys/values are
  not confused with empty entries.
• Grid-stride kernel loop (large inputs were previously truncated by the grid
  clamp).
• Null-argument guard for parity with CPU.

Tests

• CPU: test_intersect (7 cases).
• OpenCL: test_opencl_intersect (17 cases) — types, boundary keys/values,
  large inputs, negative cases, deferred (scheduled) path.