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/loopform.rs

@@ -214,3 +214,26 @@ pub fn normalize_header_phis_for_latch<'ctx>(
     }
     Ok(())
 }
+
+// Dev check: when enabled, log PHIs that live outside dispatch blocks created by LoopForm
+pub(in super::super) fn dev_check_dispatch_only_phi<'ctx>(
+    phis_by_block: &std::collections::HashMap<
+        crate::mir::BasicBlockId,
+        Vec<(crate::mir::ValueId, inkwell::values::PhiValue<'ctx>, Vec<(crate::mir::BasicBlockId, crate::mir::ValueId)>)>,
+    >,
+    loopform_registry: &std::collections::HashMap<
+        crate::mir::BasicBlockId,
+        (inkwell::basic_block::BasicBlock<'ctx>, inkwell::values::PhiValue<'ctx>, inkwell::values::PhiValue<'ctx>, inkwell::basic_block::BasicBlock<'ctx>)
+    >,
+) {
+    if std::env::var("NYASH_DEV_CHECK_DISPATCH_ONLY_PHI").ok().as_deref() != Some("1") {
+        return;
+    }
+    // Best-effort: Just report PHI presence per block when LoopForm registry is non-empty.
+    if !loopform_registry.is_empty() {
+        for (bid, phis) in phis_by_block.iter() {
+            if phis.is_empty() { continue; }
+            eprintln!("[DEV][PHI] bb={} has {} PHI(s)", bid.as_u32(), phis.len());
+        }
+    }
+}