Introduce scalafmt

[ddworak]

Summary

Introduces scalafmt to scex (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), with two deliberate differences:
  • runner.dialect = scala213 (not Scala213Source3) — scex does not compile with -Xsource:3.
  • RedundantParens is omitted from the rewrite rules. scex is existential-type-heavy, and that rule strips parens that are syntactically required around T forSome { ... } types in lambda-parameter ascriptions, producing code that no longer parses (hit in ValidationTest.scala and ScexCompilerTest.scala). Documented inline in the config.
• Adds sbt-scalafmt 2.6.1 to project/plugins.sbt.
• Reformats the whole codebase (scalafmtAll + scalafmtSbt): 123 files.
• Adds .git-blame-ignore-revs pointing at the reformat commit so git blame skips it.

Commits

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

Test plan

• sbt scalafmtCheckAll scalafmtSbtCheck passes after reformat
• sbt compile Test/compile passes (verifies the rewrite rules didn't break anything)
• CI green (no formatting gate added in this PR; build/test unaffected)

Follow-up

A separate PR will add the CI enforcement step (mirroring scala-commons / intellij-hocon).