feat(tf2): support or reject legacy TensorFlow-only descriptors

[njzjz-bot]

Summary

The unified schema exposes TensorFlow descriptor variants including loc_frame, se_a_tpe/se_a_ebd, se_a_ebd_v2/se_a_tpe_v2, and se_a_mask, while TF2 does not register matching descriptor implementations.

The current “TensorFlow” support label does not distinguish legacy TF from TF2, so users discover configurations that fail only after backend selection.

Scope

For each descriptor family, either:

• implement a TF2-compatible descriptor with shared serialization/inference semantics; or
• add backend-aware validation and documentation that explicitly limits it to legacy TF.

Avoid silent substitution with a different descriptor.

Acceptance criteria

• Every listed descriptor has an explicit TF2 support status.
• Supported descriptors construct, serialize, deserialize, and run deterministic forward/inference tests.
• Unsupported descriptors fail during backend-aware validation with a clear TF2-specific message.
• Aliases resolve consistently with the common schema.
• Documentation distinguishes legacy TF and TF2 support.

Refs #5757.

Coding agent: Codex
Codex version: codex-cli 0.144.4
Model: gpt-5.6-sol
Reasoning effort: xhigh