chore: disable additional Scala 3 specific syntax in scalafmt

Motivation:
Ensure consistent code style across the codebase by explicitly disabling
Scala 3 specific syntax features that could lead to inconsistent formatting.

Modification:
- Add allowQuestionMarkPlaceholder = false to dialectOverride
- Add allowMatchAsExpr = false to dialectOverride
- Document Scala 3 syntax restrictions in CLAUDE.md

Result:
scalafmt now enforces restrictions on Scala 3 specific syntax including
significant indentation, as import rename, * wildcard import, postfix *
vararg splices, ? placeholder, and match as expression.

Tests:
Not run - configuration only

References:
None - code style configuration

Author: He-Pin

.scalafmt.conf

@@ -81,5 +81,7 @@ runner {
     allowAsForImportRename = false
     allowStarWildcardImport = false
     allowPostfixStarVarargSplices = false
+    allowQuestionMarkPlaceholder = false
+    allowMatchAsExpr = false
   }
 }

CLAUDE.md

@@ -6,6 +6,7 @@ Before opening or updating a PR, verify:
 
 - Non-doc-only changes have directional tests.
 - Native `scalafmt` or the sbt scalafmt tasks were run for changed Scala/SBT files, or the missing tool is recorded in `Tests`.
+- Scala 3 specific syntax (significant indentation, `as` import rename, `*` wildcard import, postfix `*` vararg splices, `?` placeholder, `match` as expression) must not be used. The `.scalafmt.conf` enforces these restrictions via `dialectOverride`.
 - `sbt javafmtAll` was run with JDK 17 when relevant.
 - `sbt headerCreateAll` was run to add headers for new files. Never hand-write or invent license headers; let sbt manage them, and preserve existing copyright notices intact.
 - For copied code, the source file or external project is noted in the PR (see Licensing Rules in `AGENTS.md`).