perf: create link objects lazily

[radoering]

Another small performance improvement, which makes locking about 10 % faster in some cases.

[sourcery-ai]

Hey - I've left some high level feedback:

• With LinkCache now holding factories instead of Link instances, repeated iteration over links/links_for_version will rebuild Link objects each time; if these APIs are used frequently in hot paths, consider memoizing the constructed Links per factory or documenting that callers should cache results if they rely on reuse.
• In HTMLPage._link_cache, the comment notes that links are built eagerly due to lack of a separate filename field; if this becomes a noticeable hotspot, you might consider a lightweight helper that extracts the filename from the URL without constructing a full Link, to align with the lazy strategy introduced for the JSON source.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- With `LinkCache` now holding factories instead of `Link` instances, repeated iteration over `links`/`links_for_version` will rebuild `Link` objects each time; if these APIs are used frequently in hot paths, consider memoizing the constructed `Link`s per factory or documenting that callers should cache results if they rely on reuse.
- In `HTMLPage._link_cache`, the comment notes that links are built eagerly due to lack of a separate filename field; if this becomes a noticeable hotspot, you might consider a lightweight helper that extracts the filename from the URL without constructing a full `Link`, to align with the lazy strategy introduced for the JSON source.

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