Handle unreliable Vulkan memory budgets

[surma]

Summary

Some Vulkan drivers do not expose VK_EXT_memory_budget. In that case, node-llama-cpp can still return heap-size values via getVramState(), but live allocation accounting is not useful: used remains 0 and free === total even after loading a GPU-offloaded model.

QMD currently treats that state as reliable VRAM availability and derives embedding/rerank context parallelism from it. On PanVK/Mali-G610 this causes QMD to create too many embedding contexts, which reproduces as:

vk::CommandBuffer::end: ErrorOutOfDeviceMemory

It can also crash the process when multiple embedding contexts evaluate concurrently.

This PR adds a conservative fallback for unreliable Vulkan memory-budget reporting:

• detect the unreliable Vulkan budget shape (used === 0, free === total, no unified budget)
• cap context parallelism to 1 for that case
• use a fresh embedding context per embedding on that path while keeping the model loaded
• run embedBatch() sequentially with fresh contexts on that path
• add unit coverage for the fallback behavior

Motivation / repro notes

Observed on aarch64-linux with Mali-G610/PanVK:

• Vulkan backend loads successfully
• model offloading works
• getVramState() reports roughly the full device heap as free before and after model loading
• QMD creates its maximum embedding context pool from that bogus free-memory estimate
• 8 embedding contexts fail with VK_ERROR_OUT_OF_DEVICE_MEMORY
• reusing one embedding context for a second medium-sized input can also fail
• creating a fresh embedding context per input succeeds reliably

So the issue is not “no GPU”; it is unsafe concurrency/context reuse decisions caused by unreliable Vulkan memory accounting.

Tests

CI=true bun test test/llm.test.ts

Result: passes.

I also attempted broader local validation, but there are unrelated local environment failures in this checkout:

• bun run build fails on current main with:

  src/store.ts(2142,22): error TS2339: Property 'transaction' does not exist on type 'Database'.
  

• full CI=true bun test could not complete locally because CLI tests need better-sqlite3 native bindings, which were not built in this checkout on my aarch64-linux machine.