refactor: Gemini/ChatGPT collaborative refactoring + build fixes

Major changes:
- Split runner module: 1358→580 lines (via Gemini)
- Create new modules: dispatch.rs, selfhost.rs, pipeline.rs, pipe_io.rs
- Fix build errors from incomplete method migrations
- Add warning to CLAUDE.md about JIT/Cranelift not working
- Create interpreter.rs mode module
- Refactor loop builder into separate module

Build status:
- ✅ Executable builds successfully
- ✅ Basic execution works (tested with print)
- ⚠️ 106 warnings remain (to be cleaned up next)
- ⚠️ execute_mir_mode still in mod.rs (needs further migration)

Note: ChatGPT correctly fixed runner.execute_mir_mode() calls
that I incorrectly changed to super::modes::mir::

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

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

src/mir/builder/exprs.rs

@@ -1,6 +1,6 @@
 // Expression lowering split from builder.rs to keep files lean
-use super::{MirInstruction, ConstValue, BasicBlockId, ValueId};
-use crate::ast::{ASTNode, LiteralValue};
+use super::{MirInstruction, ConstValue, ValueId};
+use crate::ast::ASTNode;
 
 impl super::MirBuilder {
     // Main expression dispatcher