QVAC-14104 feat[api]: SDK diffusion plugin integration

[donriddo]

Re-raise of #873 onto main now that #656 (Diffusion Addon) has been merged.

What problem does this PR solve?

• The diffusion addon (QVAC-13445: Feature media generation #656) is merged but has no SDK integration — users cannot access image generation through the SDK client API
• No plugin, schemas, client helper, RPC handler, or tests exist for the diffusion engine in the SDK

How does it solve it?

• Adds sdcpp-generation plugin following established plugin architecture (definePlugin, createModel, resolveConfig, handlers)
• Adds diffusion() client API with streaming support (progressStream, outputs, stats)
• Adds Zod schemas for diffusion config, generation requests/responses, and stats
• Wires model type "sdcpp-generation" (alias "diffusion") across all SDK surfaces
• Resolves companion model paths (clipL, clipG, t5Xxl, llm, vae) via ctx.resolveModelPath()
• Adds mobile consumer support (SD 2.1 with vae_on_cpu: true)

API Changes

import { loadModel, diffusion, SD_V2_1_1B_Q8_0 } from "@qvac/sdk";

const modelId = await loadModel({
  modelSrc: SD_V2_1_1B_Q8_0,
  modelType: "diffusion",
  modelConfig: { prediction: "v" },
});

const { progressStream, outputs, stats } = diffusion({
  modelId,
  prompt: "a cat sitting on a windowsill",
  width: 512,
  height: 512,
  steps: 20,
});

for await (const { step, totalSteps } of progressStream) {
  console.log(`${step}/${totalSteps}`);
}

const buffers = await outputs;

How was it tested?

• Unit tests: 279/279 pass
• Desktop integration tests (MQTT): 13/13 diffusion tests pass
• 3 examples verified end-to-end: diffusion-simple (SD 2.1), diffusion-txt2img (FLUX), diffusion-flux2-klein (FLUX)
• FLUX model load verified on macOS arm64 (Mac Studio M3 Ultra) with diffusion-cpp 0.1.1
• Mobile consumer: SD 2.1 config with device: "gpu", threads: 4, prediction: "v", vae_on_cpu: true

Known Limitations

• img2img path removed (pending addon support in QVAC-14019: feat(diffusion): add img2img generation via in-context conditioning #884 / feature-im2im)
• Mobile diffusion tests blocked on CLI BUILTIN_PLUGINS update (PR fix[notask]: add sdcpp-generation to CLI BUILTIN_PLUGINS #1338)

PR Validation fails

[NamelsKing]

/review

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ✅ (1/1)
- 1 Team Lead OR Management approval ✅ (1/1)



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*This comment is automatically updated when reviews change.*

[NamelsKing]

/review