Address TODOs, refactor repeated lines

[mattcieslak]

There were a couple TODOs that suggested refactoring and a few lines of code that were repeated many times throughout the codebase. This PR adds some functions that reduce the repetition. Specifically:

• Centralize type dispatch: Replace ~18 copy-pasted if/else dtype dispatch chains and ~51 scattered placement-new + reinterpret_cast sites with a single detail::remap() helper. Inside TrxFile<DT>, the template parameter is already known at compile time. The runtime dispatch was unnecessary.
• Custom exception hierarchy: Replace all std::runtime_error / std::invalid_argument throws with library-specific exceptions (TrxIOError, TrxFormatError, TrxDTypeError, TrxArgumentError), all inheriting from TrxError : std::runtime_error. Existing catch(std::runtime_error) blocks continue to work.
• Public accessors: Add data(), offsets(), lengths() on ArraySequence and matrix() on MMappedMatrix.
• Make load_from_zip/load_from_directory public on TrxFile.
• Remove the #define private public hack from tests.
• RAII wrappers for libzip: detail::ZipArchive and detail::ZipFile eliminate manual zip_close/zip_fclose cleanup in error paths across all zip-handling code.
• Rewrite deepcopy(): Replace the write-to-disk-then-reload round-trip with direct _initialize_empty_trx + Eigen block copies.
• TrxStream API cleanup: set_dimensions, set_voxel_to_rasmm, set_metadata_mode, set_metadata_buffer_max_bytes now return TrxStream& for chaining.
• Replace .compare() with == across ~23 string comparisons.
• Resolve ~10 TODO comments (fixed, documented as known limitations, or removed after the underlying issue was addressed).

[codecov]

Codecov Report

❌ Patch coverage is 63.55476% with 203 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.86%. Comparing base (39a542f) to head (7caf6b3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
include/trx/trx.tpp	64.12%	113 Missing ⚠️
src/trx.cpp	41.37%	85 Missing ⚠️
include/trx/detail/zip_raii.h	78.26%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   85.25%   86.86%   +1.61%     
==========================================
  Files          13       14       +1     
  Lines        6076     5809     -267     
  Branches      946      881      -65     
==========================================
- Hits         5180     5046     -134     
+ Misses        896      763     -133     

Flag	Coverage Δ
linux	86.78% <62.93%> (+1.68%)	⬆️
macos	86.78% <63.35%> (+1.61%)	⬆️

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