fix(posts): reschedule raytracing talk to September 10

[aleiton]

The UNICEN Tandil talk moved from August 27 to September 10, 2026.

Changes

• ES / EN posts — event date updated in three places each: front-matter description, the bold intro line, and the "Cuándo y dónde" / "When and where" bullet. Now reads jueves 10 de septiembre, 2026 / Thursday, September 10, 2026 (Sept 10 is indeed a Thursday).
• Flyer — static/images/posts/raytracing-rust/flyer.png replaced in place with the new dated version, so the image URL and all markup stay unchanged.

A repo-wide sweep confirms no agosto / August references to this event remain anywhere else in content/, layouts/, or static/.

Deliberately not changed

• Front-matter date (post publication date) and the folder name — publication metadata, not the event date. The permalink is :contentbasename, so the URL is unaffected.
• The post's stated time, 17:00–19:00 hs, which disagrees with the flyer's 18hs. This mismatch predates the reschedule (the original August flyer also said 18hs), so it is left as-is here.

Follow-ups outside this PR

• The embedded YouTube live stream (LK7T9MsoluA) is still scheduled for Aug 27 and likely needs rescheduling or a new link.
• The Google Calendar event behind the "ver el evento" link still shows the old date.

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