Drop ZINC/SBT URI scheme and prune dead helpers

[jupblb]

Two small, independent cleanups that don't depend on any in-flight work.

Drop ZINC/SBT URI scheme

Scala/SBT support was dropped in #883, but the SBT-via-Zinc plumbing in the javac plugin lingered:

• UriScheme.SBT (already @Deprecated) and UriScheme.ZINC
• -build-tool:sbt argument parsing
• vf://tmp/ URI handling in SemanticdbTaskListener.absolutePathFromUri

The minimized sbt project was the last user of -build-tool:sbt. Updated it to fork javac with --add-exports flags (same trick semanticdbKotlincMinimized already uses), so the plugin sees real file paths instead of sbt's vf:// virtual-file URIs.

Remove dead helpers and unused imports

Pure dead-code removal:

• Result.{fold,map,isError,equals,hashCode,toString} — none called externally; map only called fold recursively. Improved getOrThrow/getErrorOrThrow error messages and made kind final.
• GlobalSymbolsCache.isNone — single-line wrapper for sym == null, inlined into its sole caller; dropped now-unused ElementKind import.
• 8 unused imports in AnalyzerCheckers.kt (kotlin.math.exp, FirBasicExpressionChecker, FirClassReferenceExpressionChecker, FirClassReferenceExpression, FirStatement, coneType, resolvedType).

Validation

• sbt checkAll ✅
• sbt unit/test ✅
• sbt snapshots/test ✅ (no snapshot drift)
• sbt minimized/clean minimized/compile ✅
• sbt semanticdbKotlincMinimized/clean kotlincSnapshots ✅ (no snapshot drift)