fix(robustness): improve Markdown link parsing, diagnostics, and thread safety

[lkdmc]

Summary

Three robustness improvements that prevent silent misbehaviour in edge cases.

1. Fragile Markdown link parsing in _build_attribution_display (__init__.py:951)

The source-value parser used split("](") to detect the [text](url) format. This breaks silently when the URL itself contains the two-character sequence ]( (e.g. some documentation links with anchor fragments). The check len(...split("](")) == 2 also passed spuriously for strings with a single ]( anywhere.

Fix: replace the split heuristic with re.fullmatch(r"\[([^\]]+)\]\(([^)]+)\)", ...) which correctly matches only well-formed Markdown inline links and is unambiguous regardless of URL content.

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2. Misleading diagnostic messages for imageless figures (__init__.py:510)

When a figure has no image argument, the code appended "dummy.png" to self.arguments as a placeholder, and all subsequent warning and error messages used self.arguments[0]. Every diagnostic for such a figure therefore reported "dummy.png" instead of a meaningful identifier, making it impossible to locate the offending directive.

Fix: capture _figure_id = self.arguments[0] if self.arguments else "<no image>" before the placeholder is appended, and use _figure_id in all diagnostic messages (unrecognized BibTeX key, missing/invalid license, invalid date, missing source).

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3. Thread-unsafe lazy initialisation of _untranslate_map_cache (__init__.py:1672)

parallel_read_safe = True allows Sphinx to fork worker processes that call untranslate_license() concurrently. The previous check-and-set pattern (if _untranslate_map_cache is None: _untranslate_map_cache = ...) is not atomic; two workers can both observe None and both call _load_untranslate_map() simultaneously.

Fix: protect the initialisation block with a threading.Lock using the double-checked locking pattern so that the JSON file is read at most once per process regardless of concurrency.

Test plan

• Set :source: to a URL containing ]( (e.g. [docs](https://example.com/path#section)) and confirm the link renders correctly.
• Create a figure with no image argument and an invalid license; confirm the warning names <no image> rather than dummy.png.
• Run a parallel Sphinx build (sphinx-build -j auto) with a non-English locale and confirm no exception from concurrent untranslate_license() calls.

https://claude.ai/code/session_01ApsmWPWsKBesMV1rq2VenA