Add 128-bit genome encoding layer

[Spbd1]

Motivation

• Provide a compact, deterministic 128-bit integer representation for digital genomes with stable named bit fields and deterministic decoding.
• Offer safe utilities for reading and updating bit fields and enforce the valid raw range 0 <= raw < 2**128 to avoid silent corruption.
• Allow reproducible random generation of 128-bit genomes for experiments and tests while preserving backward-compatible callable behavior for existing package-level random_genome call sites.

Description

• Add pishiegen/genome/schema.py containing FieldSpec, FIELD_SPECS, FIELD_ORDER, and constants GENOME_BITS, GENOME_MIN, and GENOME_MAX, with inclusive bit-range docstrings for each named field.
• Add pishiegen/genome/encoding.py implementing extract_bits, set_bits, validate_raw_genome, and a frozen Genome(raw: int) dataclass with validate(), get_field(), set_field(), and to_dict() methods and full validation.
• Add pishiegen/genome/decoder.py with decode_genome() that accepts either a Genome instance or a raw integer and returns the deterministic dictionary of decoded fields.
• Add pishiegen/genome/random_genome.py providing a seedable generator that returns a Genome with GENOME_BITS random bits and a small module-wrapper to keep from pishiegen.genome import random_genome call sites callable for legacy code that expects the older gene-list generator.
• Add tests/test_genome_encoding.py covering valid range checks, field extraction correctness, non-corrupting set_field, and reproducible random genomes with a seed, plus docstrings in new modules describing bit ranges and intent.

Testing

• Ran the full test suite with python -m pytest -q and PYTHONPATH=. pytest -q, and the test runs completed successfully (all tests passed).
• Added and executed tests/test_genome_encoding.py, which passed and verifies range validation, extraction/setting semantics, and seeded reproducibility.
• The repository test suite was exercised after integration and all automated tests succeeded.

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Codex Task