feat(config): add BatchGenModelConfig resolver for GLM-5.2

[Andrewxu313]

What

Add a decoupled, pure-Python config-resolution module (BatchGenModelConfig)
as the single producer of the engine's model config, replacing hand-hardcoded
values in the GLM-5 initializer that silently drifted from the checkpoint.
resolve(model_name, checkpoint_path) returns a rich BaseModelConfig
subclass via a supported-variant registry plus a per-model from_hf() mapping;
validate() fails loud and unlisted GLM-5.x variants warn loudly. The module
body is torch/engine-free and imports model configs lazily. This also adds the
GLM-5.2 name/tokenizer patterns (more-specific before GLM-5) and resets the
GLM-5.2 DSA indexer top-k carry once per decode step so shared layers never
reuse a stale value.

Why

GLM-5.2 shares the GLM-5 model graph but needs its own config identity and a
checkpoint-backed source of truth for the engine's model config. This is the
core seam the GLM-5.2 model work stacks on. GLM-only in this change; other
model families keep the existing load_config path unchanged and migrate one
per change.

Type of Change

• model
• kernel
• core
• fix
• infra
• docs

File changes

File	Δ	Note
batchgen/config/batchgen_model_config.py	add	resolver: resolve(model_name, ckpt) -> BaseModelConfig subclass; torch/engine-free
batchgen/config/model_registry.py	mod	GLM-5.2 name patterns before GLM-5; loud warning for unlisted GLM-5.x variants
batchgen/config/tokenizer_registry.py	mod	GLM-5.2 tokenizer pattern before GLM-5
batchgen/batchgen_worker.py	mod	reset GLM5AttnWrapper._dsa_prev_topk_indices once per decode step (both dispatch paths)

Checklist

• Every changed file traces to this task; the diff is surgical and one concern (§3).
• Changed files stay within the declared type's allowlist — a model/kernel PR does not touch the scheduling/scaffolding layer (§2.5–§2.6).
• The File changes table lists every changed file and matches the diff (§2.5).
• No debug_*/scratch_*/tmp_* scripts, BATCHGEN_* env-guards, stray print(), committed artifacts, or Co-Authored-By (§1/§4). (bench_* benchmarks are fine.)
• Tests added/updated under tests/; touched modules' MODULE.md updated if the public API changed (§2).
• Ran bash .github/workflows/scripts/check-pr-hygiene.sh origin/main locally; the CI hygiene check is green.

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