[codex] rename graph API terminology

[shinaoka]

Summary

Breaking terminology migration for the public graph API:

• Rename Fragment/FragmentBuilder to Graph/GraphBuilder.
• Rename value and operation identifiers/keys/references to Value* and Operation* names.
• Rename GraphOp/EvalGraphOp to GraphOperation/EvaluableGraphOperation.
• Replace OpMode::Primal/OpMode::Linear with OperationRole::Primary/OperationRole::Linearized.
• Remove OpEmitter from the public API; callers use GraphBuilder::add_operation or higher-level downstream builder traits.
• Rename public fields from op/mode to operation/role, and materialized graph fields from vals/ops to values/operations.
• Update README wording and public doctests to use the new names.

Why

Fragment and Emitter were implementation-oriented terms that leaked older terminology into downstream APIs. The new names use general graph vocabulary and align with tidu/tenferro public terminology.

Validation

• cargo test --release
• cargo test --doc --release
• cargo clippy --all-targets --release -- -D warnings
• cargo fmt --all --check
• git diff --check

Notes

This is intentionally breaking; no compatibility aliases are included.