GPT-2 GGUF export missing hidden_dim metadata (roundtrip fails)

[noahgift]

Bug Report

Source: tiny-model-ground-truth parity checker (0/59 passing)
Severity: High — blocks GPT-2 GGUF roundtrip
Related: Follow-up to #233 (GPT-2 architecture support)

Description

When reimporting a GPT-2 GGUF file (apr import model.gguf), the contract enforcement fails because hidden_dim=0:

CONTRACT ENFORCEMENT FAILED: Missing vocab_size (50257) or hidden_dim (0).
Cannot validate tensor layouts without model config. This GGUF file may be malformed.

The GGUF export for GPT-2 writes vocab_size but not hidden_dim (or writes it as 0).

Error Output

[PMAT-222] Auto-detected architecture: Gpt2
[PMAT-224] WARNING: Architecture 'GPT-2' has not been verified for inference.

error: CONTRACT ENFORCEMENT FAILED: Missing vocab_size (50257) or hidden_dim (0).
Cannot validate tensor layouts without model config.

Root Cause

The GGUF export path (apr export --format gguf) doesn't write the correct metadata key for GPT-2's hidden dimension. The GGUF metadata writer likely uses an architecture-specific key like gpt2.embedding_length that either:

1. Isn't being written during export, or
2. Isn't being read back during reimport

This may also be related to the hidden_dim=64 bug (see companion issue) — if the APR file itself stores 64 as hidden_dim, the GGUF export might be writing 64 or 0.

Reproduction

cd tiny-model-ground-truth
apr import models/gpt2-124m-int4.gguf -o models/gpt2-roundtrip.apr
# → CONTRACT ENFORCEMENT FAILED: hidden_dim (0)

Environment

• apr v0.2.16 + Int8 quantization corrupts embedding tensors (NaN/Inf + shape mismatch) #231/232/233 fixes applied
• Platform: Linux x86_64

[noahgift]

Fixed by GH-234 (commit e0e6be6) and GH-237 kaizen (commit e19c37e). Architecture detection now explicitly recognizes GPT-2, and resolve_architecture() falls back to model_type before defaulting to qwen2. GGUF export correctly writes gpt2.embedding_length instead of qwen2.embedding_length. Verified by test_gh236_architecture_fallback_uses_model_type regression test.