Add domain_tokenizer.py — DomainTokenizerBuilder (core assembler, HF integration)

Add field_tokenizers.py — Sign, MagnitudeBucket, Calendar, Categorical, DiscreteNumerical tokenizers

Add tokenizers package init

Add schemas package init

Add schema.py — DomainSchema, FieldSpec, FieldType definitions

Phase 2A: Core tokenizer library — schema, field tokenizers, composite builder, predefined schemas, 72 passing tests