Introduce scalafmt

[ddworak]

Summary

Introduces scalafmt to udash-core (config + plugin + one-shot reformat). No CI enforcement gate yet — that's a deliberate follow-up so this large mechanical diff can be reviewed on its own.

• Adds .scalafmt.conf based on the shared AVSystem config (as used in scala-commons), runner.dialect = Scala213Source3 (matches the project's -Xsource:3).
• Adds sbt-scalafmt 2.6.1 to project/plugins.sbt.
• Reformats the whole codebase (scalafmtAll + scalafmtSbt): 475 files across all modules (JS + JVM).
• Adds .git-blame-ignore-revs pointing at the reformat commit so git blame skips it. Reviewers/maintainers can opt in locally with git config blame.ignoreRevsFile .git-blame-ignore-revs.

Commits

Kept separate for review: setup (config + plugin) -> pure reformat -> blame-ignore.

Test plan

• sbt scalafmtCheckAll scalafmtSbtCheck passes after reformat (idempotent)
• sbt compile Test/compile passes across all modules — verifies the rewrite rules (RedundantBraces/RedundantParens/AvoidInfix/etc.) didn't break anything (needs ~8G heap, unrelated to this change)
• CI green (no formatting gate added in this PR; build/test unaffected)

Notes

• This adopts the AVSystem house style verbatim for consistency across our Scala repos. If the Udash maintainers prefer different settings (alignment, rewrite rules, max column), the .scalafmt.conf is the single place to adjust before this lands.

Follow-up

A separate PR will add the CI enforcement step.