feat(llvm): Major refactor - BuilderCursor全域化 & Resolver API導入

Added:
- Resolver API (resolve_i64) for unified value resolution with per-block cache
- llvmlite harness (Python) for rapid PHI/SSA verification
- Comprehensive LLVM documentation suite:
  - LLVM_LAYER_OVERVIEW.md: Overall architecture and invariants
  - RESOLVER_API.md: Value resolution strategy
  - LLVM_HARNESS.md: Python verification harness

Updated:
- BuilderCursor applied to ALL lowering paths (externcall/newbox/arrays/maps/call)
- localize_to_i64 for dominance safety in strings/compare/flow
- NYASH_LLVM_DUMP_ON_FAIL=1 for debug IR output

Key insight: LoopForm didn't cause problems, it just exposed existing design flaws:
- Scattered value resolution (now unified via Resolver)
- Inconsistent type conversion placement
- Ambiguous PHI wiring responsibilities

Next: Wire Resolver throughout, achieve sealed=ON green for dep_tree_min_string

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

@@ -13,6 +13,7 @@ mod maps;
 mod arith_ops;
 mod call;
 mod loopform;
+mod resolver;
 
 pub(super) use blocks::{create_basic_blocks, precreate_phis};
 pub(super) use flow::{emit_branch, emit_jump, emit_return};
@@ -26,3 +27,4 @@ pub(super) use arith_ops::{lower_binop, lower_unary};
 pub(super) use call::lower_call;
 pub(super) use loopform::{LoopFormContext, lower_while_loopform};
 pub(super) use loopform::normalize_header_phis_for_latch;
+pub(super) use resolver::Resolver;