Make the saturation limit convert once, in one place

The limit is EXCLUSIVE inside Jungfraujoch - the first value that is no longer a count - and NXmx
saturation_value is INCLUSIVE, the highest value that still is one. XDS OVERLOAD and the DIALS
trusted_range read it inclusively too. The write side subtracted the count and no read side added it
back, so the value fell by one on every write-read-write cycle, unbounded: four chained runs over one
dataset gave 32766, 32765, 32764, 32763. It also fed the preprocessor, so one more real count was
called saturated after each cycle.

SaturationValueFromLimit / SaturationLimitFromValue now carry the conversion, used by the writer and
by all three readers (HDF5, the lite receiver, the viewer). JFJochReaderImage's summation test moves
from > to >= in the same commit: it was silently compensating for the missing count, and correcting
one without the other would have shifted it instead.

Two more places said the wrong thing about the same pixels:

error_value was GetUnderflow(), which is -1 for an unsigned image - a value no unsigned pixel can
hold. The marker those images really carry is UINTx_MAX, and GetImageFillValue() already returned it,
so the class held two disagreeing definitions of one marker.

bit_depth_readout is now written for unsigned images only. DIALS remaps the top two codes of
2^bit_depth_readout to -1 and -2 whenever the field is present, without looking at the pixel type.
For an unsigned image those fall below underload_value and are masked, which is what we want. For a
signed one they land INSIDE the trusted range, so a saturated pixel reached DIALS as a trusted count
of -2 - on the strongest reflections. Verified with DIALS 3.27: an int32 file now masks both
sentinels. The field stays where it earns its keep, since dxtbx cannot read unsigned 32-bit without
it.

Neither the values themselves nor the wire format change. Verified against NXmx, DECTRIS SIMPLON,
Durin (Global Phasing fork), XDS and DIALS 3.27; the chained run now holds at 32766.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VfYvJT5Nb71suJCowRBn5z

reader/JFJochReaderImage.cpp

@@ -226,7 +226,7 @@ void JFJochReaderImage::AddImage(const JFJochReaderImage &other) {
             if (sum < MIN_REAL_PXL_VALUE) [[unlikely]] {
                 image[i] = ERROR_PXL_VALUE;
                 error_pixel.emplace(static_cast<int64_t>(i));
-            } else if (sum > dataset->experiment.GetSaturationLimit()) [[unlikely]] {
+            } else if (sum >= dataset->experiment.GetSaturationLimit()) [[unlikely]] {
                 image[i] = SATURATED_PXL_VALUE;
                 saturated_pixel.emplace(static_cast<int64_t>(i));
             } else {