WIP: Add division and modulus operators

[AaronWebster]

Summary

Work in progress: adds / (division) and % (modulus) operators to the Emboss expression language.

Changes span:

• Front end: tokenizer/grammar/module IR, expression bounds, type checking, and constraint validation for the new operators.
• Back end: C++ codegen support, runtime arithmetic helpers, and golden output.
• Tests: unit tests in constraints_test.py, expression_bounds_test.py, module_ir_test.py, plus an integration test (division_modulus.emb + division_modulus_test.cc) and an arithmetic runtime test.
• Docs: grammar and language reference updates.
• Formatter: format_emb.py handles the new operators.

Status

Draft — still iterating. Notable open items / things to double-check:

• Cached parser regeneration is included (large diff in cached_parser.py).
• Bounds and constraint behavior around division-by-zero / negative operands needs another pass.

Test plan

• bazel test //... clean
• Verify generated C++ for division_modulus.emb matches the golden
• Confirm formatter output for new operators
• Review division-by-zero handling in constraints and bounds