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/parser/declarations/static_def/members.rs

@@ -89,7 +89,7 @@ pub(crate) fn try_parse_method_or_field(
     while p.match_token(&TokenType::NEWLINE) { p.advance(); }
     // Parse method body; optionally use strict method-body guard when enabled
     let body = if std::env::var("NYASH_PARSER_METHOD_BODY_STRICT").ok().as_deref() == Some("1") {
-        p.parse_method_body_statements()? 
+        p.parse_method_body_statements()?
     } else {
         p.parse_block_statements()?
     };