fix: variable-length string attribute reading (Issue #14)

[kolkov]

Summary

Fixes #14 - Users couldn't read attributes from HDF5 files created by h5py.

Root causes identified and fixed:

1. V1/V2 Attribute Alignment: Name, datatype, and dataspace fields must be padded to 8-byte boundaries (per H5O_ALIGN_OLD macro in C library)

2. IsVariableString() Detection: Was checking Properties[0] & 0x0F == DatatypeString, but per HDF5 Format Spec III.A.2.4.d, variable-length string type is indicated by ClassBitField & 0x0F == 1

3. VLen String Data Format: Variable-length strings include a 4-byte length prefix before the Global Heap reference, making the total size 4 + offsetSize + 4 bytes

Changes:

• internal/core/attribute.go - Added DatatypeVarLen case to ReadValue(), fixed 8-byte alignment
• internal/core/datatype.go - Fixed IsVariableString() to check ClassBitField correctly
• internal/core/attribute_test.go - Added unit tests for vlen string attributes
• Documentation updated with correct attr.ReadValue() API

Test Results

Group '/' has 1 attributes:
  - File Attribute = 123456

Dataset: Test Dataset
Dataset 'Test Dataset' has 2 attributes:
  - Dataset Attribute 1 = Test Attribute 1
  - Dataset Attribute 2 = Test Attribute 2

• All tests pass ✅
• Linter: 0 issues ✅
• Official HDF5 Suite: 100% (378/378) ✅

Test plan

• Unit tests for IsVariableString() detection
• Unit tests for vlen attribute ReadValue() error cases
• Integration test with h5py-created file
• Official HDF5 test suite (100% pass rate maintained)
• Linter passes (0 issues)

[codecov]

Codecov Report

❌ Patch coverage is 41.17647% with 40 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/core/attribute.go	39.39%	40 Missing ⚠️

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