[codex] classify linear transpose inputs

[shinaoka]

Summary

• add a classified PrimitiveTransposeInput boundary for transpose rules
• track linearized graph local values as residual vs tangent-flow values
• pass residual values and linear values distinctly through linear_transpose
• update examples/tests for the new transpose-rule input type

Root Cause

linear_transpose normalized every linearized local input into an external value key before invoking primitive transpose rules. That erased whether a value was a residual helper or part of the tangent flow, allowing downstream rules to accidentally retain tangent-only values as runtime tensor operands.

Validation

• cargo test --workspace