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/flow.rs

@@ -73,7 +73,6 @@ pub(in super::super) fn emit_jump<'ctx, 'b>(
         BasicBlockId,
         Vec<(ValueId, PhiValue<'ctx>, Vec<(BasicBlockId, ValueId)>)>,
     >,
-    vmap: &HashMap<ValueId, BasicValueEnum<'ctx>>,
 ) -> Result<(), String> {
     // Non-sealed incoming wiring removed: rely on sealed snapshots and resolver-driven PHIs.
     let tbb = *bb_map.get(target).ok_or("target bb missing")?;
@@ -193,8 +192,6 @@ pub(in super::super) fn seal_block<'ctx, 'b>(
     >,
     // Snapshot of value map at end of each predecessor block
     block_end_values: &HashMap<BasicBlockId, HashMap<ValueId, BasicValueEnum<'ctx>>>,
-    // Fallback: current vmap (used only if snapshot missing)
-    vmap: &HashMap<ValueId, BasicValueEnum<'ctx>>,
 ) -> Result<(), String> {
     if let Some(slist) = succs.get(&bid) {
         for sb in slist {