#1945 [Bug]: GPU vision warm-up stays at 4096 tokens with an 8192 image cap

Descripción

### What happened

With `strata-vision --gpu --max-tokens 8192`, startup still encodes a fixed 2048 × 2048 dummy image. For the Qwen3.8-Flash-Next BF16 projector this warms only 4096 image tokens, even though real images can reach the larger configured cap.

A same-day build from `main` (`61b3fb5dd3f1e8ec09cf7e4e05208bc6d3c46406`) logged:

```text
strata-vision: warmed up at 4096 image tokens in 8.2 s
READY 2560
```

The encoder's Windows dedicated-GPU-memory counter was 1,699,991,552 bytes at READY. Encoding a synthetic 3840 × 2160 image then produced 8160 image tokens and raised that counter to 2,305,748,992 bytes: 605,757,440 bytes allocated after READY.

The server starts the language engine after the encoder reaches READY so it can budget its expert cache against the remaining GPU memory. A late allocation defeats that ordering. This test observed the allocation growth; it did not reproduce an OOM or paging failure.

Expected: an explicitly higher image cap should also increase the GPU prewarm size before READY.

Reproduction (substitute matching local GGUF/projector paths and a sufficiently large 3840 × 2160 PNG):

```text
strata-vision --mmproj <mmproj-Qwen3.8-Flash-Next-BF16.gguf> --model <first-text-GGUF-shard> --gpu --threads 16 --min-tokens 1024 --max-tokens 8192
ENC <large-image.png> <output.sve>
QUIT
```

### Strata version, GPU, OS

Strata 0.1.42 source at `61b3fb5`; AMD Ryzen AI Max+ 395 / Radeon 8060S (gfx1151), 128 GB unified RAM with 96 GB assigned to the GPU; Windows 11 Home build 26200; AMD driver 32.0.31032.1003. Standalone image encoder built with Vulkan SDK 1.4.350.0 and LLVM MinGW 23.1.3 (20261006), native CPU kernels. The deployed language engine uses HIP.

### Engine log

The relevant encoder log is included above. A narrow fix and paired CPU/Vulkan verification are prepared. Related Windows AMD image support is discussed in #1155 and PRs #1287 / #1789. Those threads cover encoder builds and installer discovery; this report concerns the warm-up size in the existing encoder.