Translator drops MyST label anchors like `(label)=` before headings

[mmcky]

Symptom

When translating a section that has a MyST label anchor placed immediately before its heading, the translator drops the (label)= line. The heading is translated correctly, but cross-references targeting the label become unresolved, which fails the Jupyter Book build (CI uses -W and converts warnings to errors).

Example

Upstream source (lecture-python-programming/lectures/jax_intro.md):

(jax_at_workaround)=
#### A Workaround

Translation output (lecture-python-programming.zh-cn/lectures/jax_intro.md on main, before fix):

#### 变通方法

The (jax_at_workaround)= line was silently dropped. A later sync (numpy_vs_numba_vs_jax.md) introduced a {ref}\jax_at_workaround`` cross-reference, which then failed to resolve and broke the build in PR #57.

Fix applied downstream

lecture-python-programming.zh-cn@5718949 — restored the anchor manually.

Source SHA for repro

• Source: QuantEcon/lecture-python-programming @ 450bafecd23db638602150b47f4272b98aad3146
• Target: QuantEcon/lecture-python-programming.zh-cn
• Model: claude-sonnet-4-6

Suggested fix

Preserve any (label)= lines verbatim when translating the heading that follows. These are MyST cross-reference anchors and must be kept identical to the source — the label itself is not translatable content.

This pattern is likely systematic; worth scanning past translation outputs for other dropped anchors.