refactor(compiler): Stage-B compiler simplification and test infrastructure

**Compiler Simplification (compiler_stageb.hako):**
- Remove complex fallback system (_fallback_enabled, _fallback_program)
- Remove flag parsing system (_collect_flags, _parse_signed_int)
- Streamline to single-method implementation (main only)
- Focus: parse args/env → extract main body → FlowEntry emit
- 149 lines simplified, better maintainability

**Parser Cleanup:**
- Fix trailing whitespace in members.rs (static_def)
- Add child_env module to runner/mod.rs

**Test Infrastructure (stageb_helpers.sh):**
- Enhance Stage-B test helper functions
- Better error handling and diagnostics

**Context:**
These changes were made during PHI UseBeforeDef debugging session.
Simplified compiler_stageb.hako eliminates unnecessary complexity
while maintaining core Stage-B compilation functionality.

**Impact:**
✅ Reduced Stage-B compiler complexity (-12 lines net)
✅ Clearer single-responsibility implementation
✅ Better test infrastructure support

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

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

src/runner/mod.rs

@@ -17,6 +17,7 @@ use nyash_rust::backend::llvm_compile_and_execute;
 use std::{fs, process};
 mod box_index;
 mod build;
+pub(crate) mod child_env;
 mod cli_directives;
 mod demos;
 mod dispatch;