Histogram bin centres overflow for pixels near the float64 limit #75

Found while fixing #32 (PR #74): _linear_percentile no longer overflows when interpolating between huge finite endpoints, but histogram.py's bin-centre computation still overflows for such pixel values, so the end-to-end path cannot yet reach the fixed interpolation (the #74 regression test calls _linear_percentile directly).

Fix: compute bin centres without forming an overflowing sum (e.g. lo/2 + hi/2 or lo + (hi - lo)/2 guarded like _lerp), or refuse with a named error when a centre is not representable; outputs for valid inputs must stay bitwise identical (tools/dump_outputs.py). Add an end-to-end test.