QVAC-17990 Add standalone ESRGAN upscaler API

[gabrielgrigoras-serv]

Summary

Adds a standalone ESRGAN upscaler API to @qvac/diffusion-cpp.

This allows callers to upscale existing encoded image bytes without loading or running a diffusion model.

New public API:

const { EsrganUpscaler } = require('@qvac/diffusion-cpp')

const upscaler = new EsrganUpscaler({
  files: {
    esrgan: '/absolute/path/to/RealESRGAN_x4plus_anime_6B.pth'
  },
  config: {
    upscaler_tile_size: 128
  },
  logger: console
})

await upscaler.load()

const response = await upscaler.upscale(inputImageBytes, {
  repeats: 1
})

await response.onUpdate(data => {
  // PNG bytes
}).await()

await upscaler.unload()

Additional PR details

Implemented:

• named export EsrganUpscaler
• standalone native ESRGAN model wrapper
• standalone native binding methods
• shared ESRGAN repeat-upscale helper
• shared image decode/encode helper
• PNG/JPEG byte input support
• PNG byte output
• repeats support with positive integer validation
• files.esrgan validation without requiring files.model
• CI-safe JS integration coverage
• native image codec unit coverage
• -shared backend loader between ImgStableDiffusion and EsrganUpscaler
• cancel parity between standalone and post-generation ESRGAN upscale
• ESRGAN e2e cancel coverage
• ESRGAN cross-instance coexistence coverage
• official Real-ESRGAN repo/model download references

Validation

• test:dts passed
• lint passed
• test:cpp:build passed
• test:cpp:run passed
• ESRGAN e2e integration test passed

Results:

• C++ tests: 102 ran, 100 passed, 2 skipped
• ESRGAN e2e: 5/5 tests passed, 57/57 assertions passed

Skipped C++ tests require local FLUX2 img2img assets and are unrelated to this PR.

Notes

• SDK/plugin exposure is out of scope
• docs/examples are planned under QVAC-17992 unless reviewers prefer them here

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ✅ (2/1)
- 1 Team Lead OR Management approval ✅ (1/1)



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*This comment is automatically updated when reviews change.*

[gianni-cor]

Follow-up after commits 237b776..2982390. Most behavioral items from the previous reviews landed cleanly — these five are what's still missing before this can ship.

Numbering matches the recap shared offline.

[gianni-cor]

One more observation while comparing the diffusion-cpp JS contract against qvac-lib-infer-llamacpp-llm. Not blocking — flagging for a follow-up.

[kinsta]

Preview deployments for qvac-docs-staging ⚡️

Status	Branch preview	Commit preview
✅ Ready	Visit preview	Visit preview

Commit: 1216b5281e464186727a3c3f9c65161c683dd374

Deployment ID: 1264e256-fd92-4cc6-a58a-a91bfcebc026

Static site name: qvac-docs-staging-fazwv

[gianni-cor]

/review

[gianni-cor]

/review