feat: export compiler error types for errors.As classification

[Azorlogh]

Refs EN-2833.

Ledger compiles committed scripts during Raft apply and must record compile and var-binding failures as structured audit facts. Today Compile flattens its typed errors with fmt.Errorf("%v", …), and VarsEncoder.Encode builds var failures as plain strings, so a host can only match on text.

Changes

1. Compile returns the compiler's typed error as is (CompilerError now embeds error). Messages are unchanged.
2. VarsEncoder.Encode returns MissingVariable{Name} and InvalidVariableValue{Name, Type, Raw, Cause} (Unwrap yields Cause). Messages are unchanged.
3. numscript.go aliases them with a Compiler prefix (the interpreter already exports TypeError, CannotCastToString, ExperimentalFeature, InvalidFeature), grouped as:
   • caused by the script: CompilerTypeError, CompilerInvalidUncappedSource, CompilerDuplicateRemaining, CompilerInvalidMetaPosition, CompilerCannotCastToString, CompilerCannotStoreScopedAccountInMeta, CompilerExperimentalFeature, CompilerInvalidFeature;
   • unsupported by the compiler yet: CompilerFeatureNotImplemented (only scaling today);
   • numscript defects: CompilerUnboundVar (the type checker rejects unbound variables first), CompilerInvalidScopedAccountPosition, CompilerUnsupportedNode;
   • caused by the vars: CompilerMissingVariable, CompilerInvalidVariableValue;
   • plus the CompilerError interface.
4. CompilerTypeError.Kind comes from the type checker, so its kinds are exported too: TypecheckErrorKind, TypecheckTypeMismatch, TypecheckUnboundVariable, TypecheckInvalidType, TypecheckBadArity, TypecheckUnknownFunction, TypecheckDuplicateVariable.
5. Same field-stability contract as feat: export interpreter error types for errors.As classification #215 (merged).
6. Tests in errors_test.go: every non-defect type (and every type-checker kind) is reached through CompileWithFeatureFlags or VarsEncoder.Encode, and errors.As plus key fields are checked.

Not covered: errors from ir.Typecheck / ir.Assemble after lowering. They mean the compiler emitted invalid IR (a numscript defect) and stay untyped.

The base branch was changed.