Reader: remove the background read-ahead

rugnux will mostly run on GPFS/Lustre-type storage. GPFS caches in its own
fixed-size pagepool, often smaller than a dataset, so streaming the files
ahead of the consumer can evict data before it is used; and choosing the
behaviour by the underlying filesystem is not wanted. This removes the
ReadAhead streamer of aa3fa6b9c: reader/ReadAhead.{h,cpp},
JFJochReader::StartReadAhead/NoteImageRead, the DataFiles() lists that only
it used (HDF5 reader, HDF5ImageSource, HDF5ImageLocator, CBF/marCCD/SMV),
and its start in rugnux_cli.cpp.

Kept from the same commit: the in-order CBF/marCCD/SMV header scan
(sweep::ForEachInOrder) and the 16 kB -> 256 kB CBF header probe, with its
test. They only change the order and size of reads the program makes
anyway, hold no memory and read less (cold 2400-frame CBF header scan
18 s -> 6 s on the measured HDD).

Output unchanged: myob p.hkl, p.mtz, p_P1.mtz and p_unmerged.mtz
byte-identical to the rc173 reference (17.1 s wall warm, 3.8 GB peak RSS).
Reader tests: [HDF5] 108 cases, CBF/marCCD/SMV/sweep/VDS/GetRawImage cases
all pass.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D1G8gJVAy6gp1K5Dz3NE5C

reader/HDF5ImageLocator.cpp

@@ -4,8 +4,6 @@
 #include "HDF5ImageLocator.h"
 #include "../common/JFJochException.h"
 
-#include <algorithm>
-
 namespace {
     // Coalesce consecutive single-image mappings into one contiguous range when the source and
     // virtual images stay contiguous in the same file/dataset.
@@ -103,22 +101,6 @@ HDF5ImageLocator::Location HDF5ImageLocator::Resolve(int64_t global_image) const
     return {layout_.master_file, static_cast<uint32_t>(global_image), layout_.master_filename};
 }
 
-std::vector<std::string> HDF5ImageLocator::DataFiles() const {
-    std::vector<std::string> ret;
-    if (layout_.format == FileWriterFormat::NXmxLegacy) {
-        for (const auto &f: layout_.legacy_files)
-            ret.push_back(f.path);
-    } else if (layout_.format == FileWriterFormat::NXmxVDS
-               && layout_.data_layout == HDF5DataSetLayout::VIRTUAL) {
-        for (const auto &mapping: layout_.vds_mappings)
-            if (CoversFirstChannel(mapping)
-                && std::find(ret.begin(), ret.end(), mapping.filename) == ret.end())
-                ret.push_back(mapping.filename);
-    } else if (!layout_.master_filename.empty())
-        ret.push_back(layout_.master_filename);
-    return ret;
-}
-
 std::vector<HDF5DataSourceMessage> HDF5ImageLocator::GetSourceMapping(uint64_t first_image,
                                                                       std::optional<uint64_t> image_count,
                                                                       uint64_t total_images,