Add Scala Days conference data

[CharlieLZ]

/claim #4

Summary

• Add Scala Days to the conference dataset.
• Preserve the existing README table schema.
• Use a conservative No for SEA where no public commitment to sign the Speaker Engagement Agreement was found, following the issue guidance.

Source notes

• Official Scala Days site: https://scaladays.org/
• The Scala language announcement says Scala Center partnered with Plain Schwarz for Scala Days 2026: https://www.scala-lang.org/blog/2026/03/26/scala-days-2026.html
• Scala Days CFP/rules indicate speaker ticket/accommodation handling for accepted speaker participants: https://cfp.scaladays.org/scala26/cfp and https://cfp.scaladays.org/scala26/page/cfp-rules-guidelines/

Verification

• rg -n "CodeMash|Scala Days|QCon" README.md
• awk README table pipe-count check: all table rows have 6 columns.
• git diff --check
• Source link check with curl -fsSL --connect-timeout 5 --max-time 15 -o /dev/null for all cited source URLs.

Demo

• Static data update demo/validation video: https://github.com/CharlieLZ/conference-data/releases/download/conference-data-349-demo/conference-data-349-demo.mp4

Demo note: this is a static README data update, so the PR diff is the review/demo surface.