[#177]: Increase code coverage above 94%

[steven-varga]

Objective

Increase h5cpp code coverage from 82.35% to above 94%.

Current State

• Coverage: 82.35% (812 hits / 986 lines / 30 files)
• Misses: 174 lines
• Gap to 94%: ~115 more hits needed

Plan

P0 — CMake fix + Pipeline test (~200 hits)

• Fix \test/CMakeLists.txt\ to link \h5cpp\ target (activates LZ4/ZSTD/SZIP filter paths)
• Add \test/H5Zpipeline.cpp\ — high-throughput chunk pipeline round-trip test on Linux

P1 — Small new tests for existing denominator (~200 hits)

• Add \test/H5capi.cpp\ — get_fill_value, set_extent, get_chunk_dims, get_size
• Add \test/H5Pall.cpp\ — instantiate dead property types
• Add \test/H5Sall.cpp\ — get_current_dims with stride/block/offset
• Add \test/H5Eall.cpp\ — mute/unmute, exception constructors
• Add \test/H5Ialgorithm.cpp\ — h5::ls

P2 — Error path expansion (~40 hits)

• Expand \test/H5Dio_roundtrip.cpp\ with invalid-handle throws
• Expand \test/H5Adelete.cpp\ with missing-attribute throws

P3 — Fix dead test + stream ops (~80 hits)

• Rewrite \test/H5Dio.cpp\ to do actual I/O for working types
• Add \test/H5cout.cpp\ for operator<< coverage

Expected Outcome

P0+P1+P2+P3 should push coverage to ~95%.