[ALICE3] Introduce flat LUT container for the TrackSmearer and rebase OTF code to use it

[aalkin]

@njacazio here is the final working version (clean - without test tasks) that can be integrated. It still needs #15275 in O2 to work - we should merge that on Monday.

@vkucera please disregard the linter errors here, as it rearranges a very old code. It can be brought to compliance with subsequent changes instead of trying to fix everything immediately.

[github-actions]

O2 linter results: ❌ 43 errors, ⚠️ 96 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/o2 for b5bb0d5 at 2026-04-17 08:51:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatLutEntry.h:154:35: error: 'std::ifstream' has not been declared
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatLutEntry.h:164:36: error: 'std::ifstream' has not been declared
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatLutEntry.cxx:226:13: error: no declaration matches 'o2::delphes::lutHeader_t o2::delphes::FlatLutData::PreviewHeader(std::ifstream&, const char*)'
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatLutEntry.cxx:239:13: error: no declaration matches 'o2::delphes::FlatLutData o2::delphes::FlatLutData::loadFromFile(std::ifstream&, const char*)'
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatLutEntry.h:154:35: error: 'std::ifstream' has not been declared
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatLutEntry.h:164:36: error: 'std::ifstream' has not been declared
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatTrackSmearer.cxx:105:45: error: no matching function for call to 'o2::delphes::FlatLutData::PreviewHeader(std::ifstream&, const char*)'
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/ALICE3/Core/FlatTrackSmearer.cxx:111:48: error: cannot convert 'std::ifstream' {aka 'std::basic_ifstream<char>'} to 'int&'
ninja: build stopped: subcommand failed.

Full log here.

[aalkin]

The required CI tests are done, legacy macOS issue is unrelated to this code and the linter issues, while being legitimate for the most part, should be addressed separately.