feat(phase-21.8): wire imports through extern_provider (Step 7 part 1)

**Step 7 進行中**: extern_provider 側の実装完了

Changes:
- Modified extern_provider.rs to read HAKO_MIRBUILDER_IMPORTS env var
- Both routes now call program_json_to_mir_json_with_imports():
  - Direct extern call (env.mirbuilder.emit)
  - hostbridge.extern_invoke route
- Deserializes JSON imports map and passes to MirBuilder
- Build succeeds ✅

Next: Wire collection side - set HAKO_MIRBUILDER_IMPORTS from runner

Related: #phase-21.8 MatI64/IntArrayCore integration

src/backend/mir_interpreter/handlers/extern_provider.rs

@@ -141,7 +141,21 @@ impl MirInterpreter {
                 }
                 if args.is_empty() { return Some(Err(ErrorBuilder::arg_count_mismatch("env.mirbuilder.emit", 1, args.len()))); }
                 let program_json = match self.reg_load(args[0]) { Ok(v) => v.to_string(), Err(e) => return Some(Err(e)) };
-                let res = match crate::host_providers::mir_builder::program_json_to_mir_json(&program_json) {
+
+                // Phase 21.8: Read imports from environment variable if present
+                let imports = if let Ok(imports_json) = std::env::var("HAKO_MIRBUILDER_IMPORTS") {
+                    match serde_json::from_str::<std::collections::HashMap<String, String>>(&imports_json) {
+                        Ok(map) => map,
+                        Err(e) => {
+                            eprintln!("[mirbuilder/imports] Failed to parse HAKO_MIRBUILDER_IMPORTS: {}", e);
+                            std::collections::HashMap::new()
+                        }
+                    }
+                } else {
+                    std::collections::HashMap::new()
+                };
+
+                let res = match crate::host_providers::mir_builder::program_json_to_mir_json_with_imports(&program_json, imports) {
                     Ok(s) => Ok(VMValue::String(Self::patch_mir_json_version(&s))),
                     Err(e) => Err(ErrorBuilder::with_context("env.mirbuilder.emit", &e.to_string())),
                 };
@@ -290,7 +304,20 @@ impl MirInterpreter {
                     }
                     ("env.mirbuilder", "emit") => {
                         if let Some(s) = first_arg_str {
-                            match crate::host_providers::mir_builder::program_json_to_mir_json(&s) {
+                            // Phase 21.8: Read imports from environment variable if present
+                            let imports = if let Ok(imports_json) = std::env::var("HAKO_MIRBUILDER_IMPORTS") {
+                                match serde_json::from_str::<std::collections::HashMap<String, String>>(&imports_json) {
+                                    Ok(map) => map,
+                                    Err(e) => {
+                                        eprintln!("[mirbuilder/imports] Failed to parse HAKO_MIRBUILDER_IMPORTS: {}", e);
+                                        std::collections::HashMap::new()
+                                    }
+                                }
+                            } else {
+                                std::collections::HashMap::new()
+                            };
+
+                            match crate::host_providers::mir_builder::program_json_to_mir_json_with_imports(&s, imports) {
                                 Ok(out) => Ok(VMValue::String(Self::patch_mir_json_version(&out))),
                                 Err(e) => Err(self.err_with_context("env.mirbuilder.emit", &e.to_string())),
                             }