feat(phase-21.8): extend collect_using_and_strip to return imports HashMap

**Step 6 Complete**: Extract using imports from .hako source in pipeline

Changes:
- Modified collect_using_and_strip() to return (cleaned, prelude_paths, imports)
- Build imports HashMap from seen_aliases (alias -> alias mapping)
- Updated all 6 call sites to handle new 3-tuple return type
- This provides MirBuilder with info about which names are valid static box references

Next: Wire imports through extern_provider to MirBuilder (Step 7)

Related: #phase-21.8 MatI64/IntArrayCore integration

src/runner/modes/common_util/resolve/prelude_manager.rs

@@ -169,11 +169,12 @@ impl<'a> PreludeManagerBox<'a> {
     ) -> Result<(String, Vec<String>), String> {
         // 既存のcollect_using_and_strip関数を呼び出す
         // TODO: 将来的にはUsingResolutionBox経由に置き換える
-        crate::runner::modes::common_util::resolve::strip::collect_using_and_strip(
+        let (cleaned, prelude_paths, _imports) = crate::runner::modes::common_util::resolve::strip::collect_using_and_strip(
             &self.runner,
             code,
             filename,
-        )
+        )?;
+        Ok((cleaned, prelude_paths))
     }
 
     /// 🔧 テキストを正規化するにゃ！

src/runner/modes/common_util/resolve/strip.rs

@@ -1,8 +1,9 @@
 use crate::runner::NyashRunner;
 
 /// Collect using targets and strip using lines (no inlining).
-/// Returns (cleaned_source, prelude_paths) where `prelude_paths` are resolved
-/// file paths to be parsed separately and AST-merged (when `NYASH_USING_AST=1`).
+/// Returns (cleaned_source, prelude_paths, imports) where:
+/// - `prelude_paths` are resolved file paths to be parsed separately and AST-merged (when `NYASH_USING_AST=1`)
+/// - `imports` is a HashMap mapping alias names to box types (for MirBuilder resolution)
 ///
 /// Notes
 /// - This function enforces profile policies (prod: disallow file-using; only
@@ -13,9 +14,9 @@ pub fn collect_using_and_strip(
     runner: &NyashRunner,
     code: &str,
     filename: &str,
-) -> Result<(String, Vec<String>), String> {
+) -> Result<(String, Vec<String>, std::collections::HashMap<String, String>), String> {
     if !crate::config::env::enable_using() {
-        return Ok((code.to_string(), Vec::new()));
+        return Ok((code.to_string(), Vec::new(), std::collections::HashMap::new()));
     }
     let using_ctx = runner.init_using_context();
     let prod = crate::config::env::using_is_prod();
@@ -421,7 +422,13 @@ pub fn collect_using_and_strip(
         with_marker.push_str(&out);
         out = with_marker;
     }
-    Ok((out, prelude_paths))
+    // Phase 21.8: Build imports map from seen_aliases (alias -> alias for now)
+    // This provides the MirBuilder with information about which names are valid static box references
+    let mut imports = std::collections::HashMap::new();
+    for (alias, _) in seen_aliases.iter() {
+        imports.insert(alias.clone(), alias.clone());
+    }
+    Ok((out, prelude_paths, imports))
 }
 
 /// Profile-aware prelude resolution wrapper (single entrypoint).
@@ -435,7 +442,7 @@ pub fn resolve_prelude_paths_profiled(
     filename: &str,
 ) -> Result<(String, Vec<String>), String> {
     // First pass: strip using from the main source and collect direct prelude paths
-    let (cleaned, direct) = collect_using_and_strip(runner, code, filename)?;
+    let (cleaned, direct, _imports) = collect_using_and_strip(runner, code, filename)?;
     // Recursively collect nested preludes (DFS) for both AST/text merges.
     // Rationale: even when we merge via text, nested `using` inside preludes
     // must be discovered so that their definitions are present at runtime
@@ -469,7 +476,7 @@ pub fn resolve_prelude_paths_profiled(
         }
         let src = std::fs::read_to_string(&real_path)
             .map_err(|e| format!("using: failed to read '{}': {}", real_path, e))?;
-        let (_cleaned, nested) = collect_using_and_strip(runner, &src, &real_path)?;
+        let (_cleaned, nested, _nested_imports) = collect_using_and_strip(runner, &src, &real_path)?;
         for n in nested.iter() {
             dfs(runner, n, out, seen)?;
         }
@@ -556,7 +563,7 @@ pub fn parse_preludes_to_asts(
         }
         let src = std::fs::read_to_string(prelude_path)
             .map_err(|e| format!("using: error reading {}: {}", prelude_path, e))?;
-        let (clean_src, _nested) = collect_using_and_strip(runner, &src, prelude_path)?;
+        let (clean_src, _nested, _nested_imports) = collect_using_and_strip(runner, &src, prelude_path)?;
 
         // IMPORTANT: Do not attempt to AST-parse .hako preludes here.
         // .hako is Hakorune surface, not Nyash AST. VM/VM-fallback paths
@@ -766,7 +773,7 @@ pub fn merge_prelude_text(
         }
         let src = std::fs::read_to_string(path)
             .map_err(|e| format!("using: failed to read '{}': {}", path, e))?;
-        let (_cleaned, nested) = collect_using_and_strip(runner, &src, path)?;
+        let (_cleaned, nested, _nested_imports) = collect_using_and_strip(runner, &src, path)?;
         for n in nested.iter() {
             dfs_text(runner, n, out, seen)?;
         }
@@ -802,7 +809,7 @@ pub fn merge_prelude_text(
             .map_err(|e| format!("using: failed to read '{}': {}", path, e))?;
 
         // Strip using lines from prelude and normalize
-        let (cleaned_raw, _nested) = collect_using_and_strip(runner, &content, path)?;
+        let (cleaned_raw, _nested, _nested_imports) = collect_using_and_strip(runner, &content, path)?;
         let mut cleaned = normalize_text_for_inline(&cleaned_raw);
         // Hako-friendly normalize for preludes: always strip leading `local ` at line head
         // when the prelude is a .hako (or looks like Hako code). This prevents top-level