bridge/json_v0: split expr lowering; add Ternary/Peek lowering + AST; selfhost Peek JSON emit; add selfhost Peek smoke; warning cleanup in lowering/optimizer/verification

- Split expr lowering into ; route calls from stmt lowering
- Implement ternary/peek lowering (MIR13 PHI-off=Copy, PHI-on=Phi)
- Extend JSON v0 AST (ExprV0::{Ternary,Peek}, PeekArmV0)
- Selfhost parser_box: emit Peek JSON; add Stage-2 'Peek basic' smoke
- Reduce warnings: remove unused imports/vars in several modules
- current_task: update plan for legacy VM/Interpreter offboarding

src/mir/builder/lifecycle.rs

@@ -1,7 +1,4 @@
-use super::{
-    BasicBlockIdGenerator, EffectMask, FunctionSignature, MirFunction, MirInstruction, MirModule,
-    MirType, ValueId,
-};
+use super::{EffectMask, FunctionSignature, MirFunction, MirInstruction, MirModule, MirType, ValueId};
 use crate::ast::ASTNode;
 
 // Lifecycle routines extracted from builder.rs
@@ -106,4 +103,3 @@ impl super::MirBuilder {
         Ok(module)
     }
 }
-