Site: the posts link to each other wherever they refer to one; check_site.py checks every link of the site

"The last post" and "the first post" in the LoRa, Gemini and S1 posts are
now links. check_site.py follows every link to another page of the site and
the #fragment it names, so a broken one fails the Site job.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

site/content/devlog/roro9stack-s1/index.md

@@ -57,11 +57,11 @@ So the backlog got sorted into milestones, and the first one is the boring kind:
 
 ## The card that never said no
 
-The last post left it here: about once in five uploads, a write to the card failed, the retry code papered over the hole with zeros, and I'd fixed the papering but not the failing. The card "refuses a write now and then", I wrote, like it was weather.
+[The last post](/devlog/roro9stack-lora/) left it here: about once in five uploads, a write to the card failed, the retry code papered over the hole with zeros, and I'd fixed the papering but not the failing. The card "refuses a write now and then", I wrote, like it was weather.
 
 ### Four wrong guesses
 
-**The radio.** It shares the card's SPI bus, so it was the obvious suspect. Cleared in the last post: the failures came with the radio asleep too.
+**The radio.** It shares the card's SPI bus, so it was the obvious suspect. Cleared in [the last post](/devlog/roro9stack-lora/): the failures came with the radio asleep too.
 
 **A timeout.** The driver waits at most 500 ms for a busy card and then gives up, silently. Cards do pause for housekeeping. So I timed every write. Good ones took up to 107 ms. The failing one came back after **4 ms**. Nothing waits 500 ms in 4 ms.