QVAC-22218 feat[api]: add Ideogram 4 diffusion support

[maxim-smotrov]

🎯 What problem does this PR solve?

• SDK users cannot load Ideogram 4's split model layout because the unconditional diffusion companion cannot be supplied to the diffusion plugin.

📝 How does it solve it?

• Adds uncondModelSrc to the public SD.cpp configuration and resolves it with the required Qwen3-VL and VAE companions.
• Validates the Ideogram-only layout and wires resolved files into @qvac/diffusion-cpp 0.14.0.
• Adds a runnable Ideogram 4 generation example.

🧪 How was it tested?

• Bare plugin tests cover schema acceptance, companion resolution, source-field stripping, and addon wiring.
• Error-path tests cover unsupported modes and missing LLM or VAE companions.

🔌 API Changes

A new uncondModelSrc param is required to use with Ideogram.

const modelId = await loadModel({
  modelSrc: IDEOGRAM_MODEL_URL,
  modelType: 'sdcpp-generation',
  modelConfig: {
    llmModelSrc: QWEN3_VL_MODEL_URL,
    vaeModelSrc: VAE_MODEL_URL,
    uncondModelSrc: IDEOGRAM_UNCOND_MODEL_URL
  }
})

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[github-actions]

Review Status

Current Status: ❌ PENDING
Approvals so far: none

Pending reviews: Needs 1 Management or Team Lead, and 1 more from Management, Team Lead, or Member.

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QVAC E2E — windows — ✅ all tests passed (106/106, 676s)

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QVAC E2E — ios — ✅ all tests passed (93/106, 1123s)

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