[Bug]: Granite4 Vision mmproj with downsample_window_side=0 crashes at load (integer division by zero in warmup graph build)

Start at tools/mtmd/models/granite4-vision.cpp and the GRANITE4_VISION branch of clip.cpp, where the downsample sides are read before graph warmup. Reproduce with gen_mmproj_granite4.py using window_side=0, then verify the loader rejects it without reaching the division sites; the window_side=8 control should still load and encode.

Git commit

The bug is reachable at current HEAD (ad1de39e, 2026-08-15) and was introduced by 64086f2b "model, mtmd: Granite4 Vision (#23545)".

Operating system

Windows (0xC0000094) and Linux (SIGFPE), x86_64. Prebuilt CPU backend. No GPU.

Steps to reproduce the bug

1. Craft a minimal granite4-vision mmproj GGUF where clip.vision.proj.sample.window_side = 0 (u32, read with no validation — load_hparams GRANITE4_VISION branch, b10252 clip.cpp:1858-1859). Generator (52 F32 tensors, key dims: v.position_embd.weight ne = (n_embd, side²), v.proj_blk.0.img_pos (n_embd, window_side², n_windows), v.proj_blk.0.query (n_embd, query_side², 1)): python3 gen_mmproj_granite4.py g4w0.gguf 0 384. Control with window_side=8 loads and encodes fine.
2. Load it with mtmd_init_from_file (default warmup=true).

Observed (Windows, LM Studio backend mtmd.dll):

[chk] params: use_gpu=0 warmup=1 n_threads=4
... load_tensors: loaded 53 tensors ...
[log INFO] get_dummy_batch: warmup with image size = 384 x 384
[VEH] exception 0xC0000094 at rip=... module=mtmd.dll rva=B2D56

gdb on Linux, b10252 Debug build:

Program received signal SIGFPE, Arithmetic exception.
#0 clip_graph_granite4_vision::build_block (..., image_side=24, window_side=0, query_side=4, ...)
   at tools/mtmd/models/granite4-vision.cpp:63        # const int n = image_side / window_side;
#1 ... #2 reserve_compute_meta ... #7 warmup ... #8 clip_init #9 mtmd_init_from_file

Same GGUF crashes HEAD ad1de39e at granite4-vision.cpp:87 (same line moved). Other division sites with the same divisor: clip.cpp:4043 (clip_n_output_tokens), clip.cpp:5135 (win_idx upload), HEAD granite4-vision.cpp:388 anyres path.

Expected behavior

load_hparams should reject zero downsample sides:

if (hparams.downsample_window_side == 0 || hparams.downsample_query_side == 0) {
    throw std::runtime_error("granite4_vision: downsample_window_side/query_side must be non-zero");
}

Other

• Integer divide-by-zero — DoS, not memory corruption.
• Crash happens at load time (warmup graph build), before any user image; in GUI apps built on mtmd (e.g. LM Studio), merely mounting such an mmproj crashes the process.
• PoC generator gen_mmproj_granite4.py available; happy to attach.