feat(mir-builder): add using imports support to json_v0_bridge

Phase 21.8 foundation for MatI64/IntArrayCore integration

Changes:
- Add `imports: HashMap<String, String>` to BridgeEnv
- Extend MapVars::resolve() to check imports and create static box references
- Add BridgeEnv::with_imports() to initialize with using imports map
- Add parse_json_v0_to_module_with_imports() to json_v0_bridge
- Add program_json_to_mir_json_with_imports() to mir_builder.rs
- Maintain backward compatibility via empty HashMap defaults

Next: Wire using extraction from pipeline and test with MatI64

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/runner/json_v0_bridge/mod.rs

@@ -4,8 +4,13 @@ mod lowering;
 
 use ast::{ProgramV0, StmtV0};
 use lowering::lower_program;
+use std::collections::HashMap;
 
 pub fn parse_json_v0_to_module(json: &str) -> Result<crate::mir::MirModule, String> {
+    parse_json_v0_to_module_with_imports(json, HashMap::new())
+}
+
+pub fn parse_json_v0_to_module_with_imports(json: &str, imports: HashMap<String, String>) -> Result<crate::mir::MirModule, String> {
     let prog: ProgramV0 =
         serde_json::from_str(json).map_err(|e| format!("invalid JSON v0: {}", e))?;
     if crate::config::env::cli_verbose() {
@@ -19,7 +24,7 @@ pub fn parse_json_v0_to_module(json: &str) -> Result<crate::mir::MirModule, Stri
     if prog.version != 0 || prog.kind != "Program" {
         return Err("unsupported IR: expected {version:0, kind:\"Program\"}".into());
     }
-    lower_program(prog)
+    lower_program(prog, imports)
 }
 
 pub fn parse_source_v0_to_module(input: &str) -> Result<crate::mir::MirModule, String> {