fix(llvm): MapBox core-first implementation with plugin fallback by ChatGPT

Implemented elegant solution for MapBox as core box with plugin fallback:

1. Core-first Strategy:
   - Removed MapBox type_id from nyash_box.toml
   - MapBox now uses env.box.new fallback (core implementation)
   - Consistent with self-hosting goals

2. Plugin Fallback Option:
   - Added NYASH_LLVM_FORCE_PLUGIN_MAP=1 environment variable
   - Allows forcing MapBox to plugin path when needed
   - Preserves flexibility during transition

3. MIR Type Inference:
   - Added MapBox method type inference (size/has/get)
   - Ensures proper return type handling

4. Documentation:
   - Added core vs plugin box explanation in nyrt
   - Clarified the transition strategy

This aligns with Phase 15 goals where basic boxes will eventually
be implemented in Nyash itself for true self-hosting.

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

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

src/mir/builder.rs

@@ -125,6 +125,10 @@ impl MirBuilder {
                         | ("StringBox", "toUpper")
                         | ("StringBox", "toLower") => Some(super::MirType::String),
                         ("ArrayBox", "length") => Some(super::MirType::Integer),
+                        // Core MapBox minimal inference (core-first)
+                        ("MapBox", "size") => Some(super::MirType::Integer),
+                        ("MapBox", "has") => Some(super::MirType::Bool),
+                        ("MapBox", "get") => Some(super::MirType::Box("Any".to_string())),
                         _ => None,
                     };
                     if let Some(mt) = inferred {