QVAC-14576 chore[bc]: remove Opus NMT engine from SDK API surface

[RamazTs]

What problem does this PR solve?

Opus NMT models have been deprecated and removed from the registry. The SDK still exposes Opus-specific schemas, types, naming utilities, examples, and test resources that reference models that no longer exist.

How does it solve it?

Remove all Opus-specific API surface from the SDK:

• "Opus" literal from NMT_ENGINES and opusConfigSchema from the discriminated union
• MARIAN_LANGUAGES array and MarianLanguage type
• generateNmtOpusName from the model naming pipeline
• translation-opus.ts example and Opus references in translation-stream.ts, plugins.ts
• Marian Opus test definitions, consumer resource configs, and executor references

Model registry (models.ts) is not touched — the SDK team will sync via bun update-models before release.

Breaking changes

BEFORE:

import { MARIAN_OPUS_EN_IT_Q4_0 } from "@qvac/sdk";

const modelId = await loadModel({
  modelSrc: MARIAN_OPUS_EN_IT_Q4_0,
  modelType: "nmt",
  modelConfig: {
    engine: "Opus",
    from: "en",
    to: "it",
  },
});

AFTER:

import { BERGAMOT_EN_ES } from "@qvac/sdk";

const modelId = await loadModel({
  modelSrc: BERGAMOT_EN_ES,
  modelType: "nmt",
  modelConfig: {
    engine: "Bergamot",
    from: "en",
    to: "es",
  },
});

How was it tested?

• bun run test:unit — all pass
• Verified no remaining Opus references in schemas, naming, examples, or tests

Cherry-pick to release-sdk-0.9.0 after merge to main.

[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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