🏗️ Refactor: Major LLVM codegen modularization + Phase 15 docs cleanup + Phase 21 DDD concept

## LLVM Codegen Refactoring (by ChatGPT5)
- Split massive boxcall.rs into focused submodules:
  - strings.rs: String method optimizations (concat, length)
  - arrays.rs: Array operations (get, set, push, length)
  - maps.rs: Map operations (get, set, has, size)
  - fields.rs: getField/setField handling
  - invoke.rs: Tagged invoke implementation
  - marshal.rs: Helper functions for marshaling
- Improved code organization and maintainability
- No functional changes, pure refactoring

## Phase 15 Documentation Cleanup
- Restructured phase-15 folder:
  - implementation/: Technical implementation docs
  - planning/: Planning and sequence docs
  - archive/: Redundant/old content
- Removed duplicate content (80k→20k line reduction mentioned 5 times)
- Converted all .txt files to .md for consistency
- Fixed broken links in README.md
- Removed redundant INDEX.md

## Phase 21: Database-Driven Development (New)
- Revolutionary concept: Source code in SQLite instead of files
- Instant refactoring with SQL transactions
- Structured management of boxes, methods, dependencies
- Technical design with security considerations
- Vision: World's first DB-driven programming language

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

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

src/backend/llvm/compiler/codegen/instructions/mod.rs

@@ -6,6 +6,10 @@ mod boxcall;
 mod arith;
 mod mem;
 mod consts;
+mod strings;
+mod arrays;
+mod maps;
+mod arith_ops;
 
 pub(super) use blocks::{create_basic_blocks, precreate_phis};
 pub(super) use flow::{emit_branch, emit_jump, emit_return};
@@ -15,4 +19,4 @@ pub(super) use boxcall::lower_boxcall;
 pub(super) use arith::lower_compare;
 pub(super) use mem::{lower_load, lower_store};
 pub(super) use consts::lower_const;
-
+pub(super) use arith_ops::{lower_binop, lower_unary};