test(serdes): add Wave 4 complex scenario tests

Add comprehensive complex scenario tests for DSDL serialization:

Task 10 - Non-byte-aligned primitive arrays (7 tests):
- bool[N] arrays with LSB-first bit packing
- uint3[4], uint5[3] sub-byte arrays
- Variable-length sub-byte arrays
- Mixed sub-byte struct {uint3, bool, uint5}
- Verified bit patterns: bool[8] [T,F,T,F,T,F,T,F] → 0x55

Task 11 - Union variant scaling + tag width (6 tests):
- Union tag width boundaries: 256 variants → 8-bit, 257 → 16-bit
- Tag width formula verification: 2^ceil(log2(max(8, (n-1).bit_length())))
- Roundtrip all variants for small unions {3, 4}
- Roundtrip variant 0 and max for large unions {256, 257}

Task 12 - Complex nested type roundtrips (8 tests):
- Deep nesting: 3-level and 4-level struct→struct→struct
- Arrays of composites: struct[N], union[N]
- Union with struct variants
- Mixed patterns: struct→union→struct, struct→array→struct
- Complex combined nesting

Task 13 - Mixed alignment, defaults, API edge cases (10 tests):
- Mixed alignment: byte + sub-byte fields (bit-packed)
- Alignment rules: primitives=1 (bit-packed), composites=8 (byte-aligned)
- Default value handling: all defaults, partial defaults
- API edge cases: empty struct, single-field struct, single-variant union
- Type coercions: int→float, list→tuple
- Error handling: invalid types, out-of-range values

All 309 tests pass (91 original + 218 new).

Author: pavel-kirienko