feat(llvm): Implement Context Boxing pattern for cleaner APIs

Major improvement to reduce parameter explosion (15+ args → 3-4 contexts):
- Add LowerFnCtx/BlockCtx for grouping related parameters
- Add lightweight StrHandle/StrPtr newtypes for string safety
- Implement boxed API wrappers for boxcall/fields/invoke
- Add dev checks infrastructure (NYASH_DEV_CHECK_DISPATCH_ONLY_PHI)

Key achievements:
- lower_boxcall: 16 args → 7 args via boxed API
- fields/invoke: Similar parameter reduction
- BuilderCursor discipline enforced throughout
- String handle invariant: i64 across blocks, i8* only at call sites

Status:
- Internal migration in progress (fields → invoke → marshal)
- Full cutover pending due to borrow checker constraints
- dep_tree_min_string.o generation successful (sealed=ON)

Next: Complete internal migration before flipping to boxed APIs

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

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

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

@@ -0,0 +1,16 @@
+use inkwell::values::{IntValue, PointerValue};
+
+/// Lightweight newtypes for string representations used in lowering.
+/// StrHandle crosses basic blocks; StrPtr is created at call sites within the same block.
+pub struct StrHandle<'ctx>(pub IntValue<'ctx>);
+pub struct StrPtr<'ctx>(pub PointerValue<'ctx>);
+
+impl<'ctx> StrHandle<'ctx> {
+    #[inline]
+    pub fn as_i64(&self) -> IntValue<'ctx> { self.0 }
+}
+
+impl<'ctx> From<PointerValue<'ctx>> for StrPtr<'ctx> {
+    fn from(p: PointerValue<'ctx>) -> Self { Self(p) }
+}
+