🔧 Add sealed SSA mode for PHI debugging (ChatGPT5)

Added NYASH_LLVM_PHI_SEALED env var to toggle PHI wiring modes:
- NYASH_LLVM_PHI_SEALED=0 (default): immediate PHI wiring
- NYASH_LLVM_PHI_SEALED=1: sealed SSA style (wire after block completion)
- Added seal_block() function for deferred PHI incoming setup
- Enhanced PHI tracing with NYASH_LLVM_TRACE_PHI=1

This helps debug 'phi incoming value missing' errors by
comparing immediate vs sealed wiring approaches.

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

@@ -1,5 +1,5 @@
 mod blocks;
-mod flow;
+pub mod flow;
 mod externcall;
 mod newbox;
 mod boxcall;