feat(translate): implement JSX placeholder mask/restore to prevent LLM corruption

[luandro]

Background

The translation pipeline currently protects JSX/MDX attribute syntax only via prompt instructions. The integration test (scripts/eval/jsx-preservation.integration.test.ts) explicitly documents that this is non-deterministic:

"Before that implementation [scripts/shared/jsxPlaceholders.ts] exists, these tests will fail non-deterministically because the LLM can corrupt JSX attribute syntax"

The file scripts/shared/jsxPlaceholders.ts is referenced as the intended fix but does not yet exist.

What to implement

A jsxPlaceholders.ts module (analogous to localeImagePlaceholders.ts) that:

1. Before translation: scans the markdown for JSX blocks and attributes (e.g. <Tabs>, <TabItem value="android">, MDX import/export statements) and replaces them with stable opaque tokens that the LLM will not alter.
2. After translation: restores the original JSX from the tokens.

The mask/restore approach makes JSX preservation deterministic regardless of LLM behaviour.

Acceptance criterion

The existing integration tests in scripts/eval/jsx-preservation.integration.test.ts should pass reliably when RUN_LIVE_TRANSLATION_TESTS=1 without the prompt-only protection being the sole guard.