feat(env): Phase 71-73 - SSA fix + Stage-3 ENV consolidation

## Phase 71-SSA: StageBDriverBox birth warning 解消
- Fixed false-positive dev verify warning for static boxes
- StageBDriverBox is a static box, so it doesn't follow NewBox→birth pattern
- Modified lifecycle.rs to skip StageBDriverBox from birth() requirement

## Phase 73-A: Stage-3 legacy ENV 統一化
- Consolidated NYASH_PARSER_STAGE3 and HAKO_PARSER_STAGE3 → NYASH_FEATURES=stage3
- Updated 20 test files (46 direct replacements)
- Special handling for parser_stage3.rs compat layer and mir_static_main_args_loop.rs
- All test files now use unified NYASH_FEATURES=stage3

## Phase 72-73: ENV inventory documented
- Created phase72-73-env-inventory.md with complete usage analysis
- Identified 113 direct ENV reads requiring SSOT consolidation
- Prioritized Phase 72 (JoinIR EXPERIMENT SSOT) and Phase 73 (Stage-3 cleanup)

## Phase 74-SSA: Minimal reproduction for static box delegation
- Created parser_box_minimal.hako and ssa_static_delegation_min.hako
- Investigated spawn failure in selfhost compiler (arguments too long)
- Root cause: NYASH_NY_COMPILER_EMIT_ONLY=1 defaults to emit-only mode

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

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

src/tests/mir_pure_e2e_vm.rs

@@ -31,7 +31,7 @@ return (new StringBox("Hello")).length()
     /// Minimal smoke for MirInstruction::DebugLog
     #[test]
     fn mir_debuglog_minimal_printer_and_verifier() {
-        std::env::set_var("NYASH_PARSER_STAGE3", "1");
+        std::env::set_var("NYASH_FEATURES", "stage3");
         std::env::set_var("NYASH_PARSER_ALLOW_SEMICOLON", "1");
         std::env::set_var("NYASH_MIR_DEBUG_LOG", "1");