fix(parser): Stage-1 CLI infinite loop - MirBuilder-friendly refactoring (Part 1/3)

**Problem**: Stage-1 CLI hits VM step budget (infinite loops) in multiple functions
- ParserBox.parse_program2: bb2853→bb2854 loop
- StringHelpers.skip_ws: bb488→bb491 loop

**Root Cause**: MirBuilder bug with `loop(cont==1)` + nested `if-else` pattern
- PHI instructions with self-references
- Loop exit blocks jumping back to header instead of continuation

**Solution**: Refactor to MirBuilder-friendly pattern
```hako
// Before (causes infinite loop):
loop(cont == 1) {
  if guard > max { return j } else { guard++ }
  if condition { action } else { cont = 0 }
}

// After (MirBuilder-friendly):
loop(j < n) {
  if condition { j++; continue }
  break
}
```

**Changes**:
1. parser_box.hako: parse_program2 main loop refactored
   - 7 sections: ws→EOF/guard→parse_stmt2→progress guard→trace→semicolons→emit
   - Unconditional parse_stmt2 execution (no nested if-else)
   - Explicit `break` for loop exits

2. string_helpers.hako: StringHelpers.skip_ws simplified
   - Removed `cont` flag and guard counter
   - Direct `loop(j < n)` with `continue/break`

3. vm.rs, vm_fallback.rs: RUST_MIR_DUMP_PATH support
   - Enables offline MIR analysis for debugging

**Progress**:
- ✅ parse_program2: infinite loop fixed
- ✅ StringHelpers.skip_ws: infinite loop fixed
- ⏳ ParserIdentScanBox.scan_ident: next target (bb985 loop)

**Testing**:
- MIR dumps generated successfully (116K+ lines)
- ws_init loop completes
- parse_program2 progresses further
- Still hits infinite loop in scan_ident (Part 2 needed)

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

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

lang/src/shared/common/string_helpers.hako

@@ -172,14 +172,13 @@ static box StringHelpers {
     local s = "" + src
     local n = s.length()
     local j = i
-    local cont = 1
-    local guard = 0
-    local max = 100000
-    loop(cont == 1) {
-      if guard > max { return j } else { guard = guard + 1 }
-      if j < n {
-        if me.is_space(s.substring(j, j+1)) { j = j + 1 } else { cont = 0 }
-      } else { cont = 0 }
+    // Simplified loop structure for MirBuilder compatibility
+    loop(j < n) {
+      if me.is_space(s.substring(j, j+1)) {
+        j = j + 1
+        continue
+      }
+      break
     }
     return j
   }