fix(parser): Part 5 - eliminate infinite loops in parse_string2 and scan_with_quote

Problem: Nested escape sequence processing in string parsing functions caused
infinite loops due to missing continue statements, allowing fallthrough to the
else block which corrupted the position counter.

Fixed functions:
- parser_expr_box.hako::parse_string2 (lines 22-83)
- parser_string_scan_box.hako::scan_with_quote (lines 11-101)

Changes:
- Flattened nested 'else { if ... }' chains to flat 'if' statements
- Added explicit 'continue' after each escape sequence processing
- Prevents fallthrough to position increment code

Verification:
✅ NYASH_USE_STAGE1_CLI=1 STAGE1_EMIT_PROGRAM_JSON=1 HAKO_VM_MAX_STEPS=2000000
   apps/tests/minimal_ssa_skip_ws.hako
✅ No budget exceeded
✅ Program JSON v0 output to stdout
✅ Exit code: 0

MirBuilder-friendly pattern applied consistently with Parts 1-4.
Stage-1 CLI infinite loop elimination COMPLETE!

lang/src/compiler/parser/expr/parser_expr_box.hako

@@ -39,18 +39,44 @@ static box ParserExprBox {
 
       if ch == "\\" && j + 1 < n {
         local nx = src.substring(j+1, j+2)
-        if nx == "\"" { out = out + "\"" j = j + 2 }
-        else { if nx == "\\" { out = out + "\\" j = j + 2 }
-        else { if nx == "n" { out = out + "\n" j = j + 2 }
-        else { if nx == "r" { out = out + "\r" j = j + 2 }
-        else { if nx == "t" { out = out + "\t" j = j + 2 }
-        else { if nx == "u" && j + 5 < n { out = out + src.substring(j, j+6) j = j + 6 }
-        else { out = out + nx j = j + 2 } } } } } }
-      } else {
+        if nx == "\"" {
+          out = out + "\""
+          j = j + 2
+          continue
+        }
+        if nx == "\\" {
+          out = out + "\\"
+          j = j + 2
+          continue
+        }
+        if nx == "n" {
+          out = out + "\n"
+          j = j + 2
+          continue
+        }
+        if nx == "r" {
+          out = out + "\r"
+          j = j + 2
+          continue
+        }
+        if nx == "t" {
+          out = out + "\t"
+          j = j + 2
+          continue
+        }
+        if nx == "u" && j + 5 < n {
+          out = out + src.substring(j, j+6)
+          j = j + 6
+          continue
+        }
+        out = out + nx
+        j = j + 2
+        continue
+      }
+
       out = out + ch
       j = j + 1
     }
-    }
 
     ctx.gpos_set(j)
     return "{\"type\":\"Str\",\"value\":\"" + ctx.esc_json(out) + "\"}"

lang/src/compiler/parser/scan/parser_string_scan_box.hako

@@ -35,48 +35,67 @@ static box ParserStringScanBox {
         if nx == "\\" {
           out = out + "\\"
           j = j + 2
-        } else { if nx == "\"" {
+          continue
+        }
+        if nx == "\"" {
           out = out + "\""
           j = j + 2
-        } else { if nx == "'" {
+          continue
+        }
+        if nx == "'" {
           out = out + "'"
           j = j + 2
-        } else { if nx == "/" {
+          continue
+        }
+        if nx == "/" {
           out = out + "/"
           j = j + 2
-        } else { if nx == "b" {
+          continue
+        }
+        if nx == "b" {
           // Backspace (0x08) - for MVP, skip (empty string)
           out = out + ""
           j = j + 2
-        } else { if nx == "f" {
+          continue
+        }
+        if nx == "f" {
           // Form feed (0x0C) - for MVP, skip (empty string)
           out = out + ""
           j = j + 2
-        } else { if nx == "n" {
+          continue
+        }
+        if nx == "n" {
           out = out + "\n"
           j = j + 2
-        } else { if nx == "r" {
+          continue
+        }
+        if nx == "r" {
           // FIX: \r should be CR (0x0D), not LF (0x0A)
           // Keep as "\r" literal for MVP
           out = out + "\r"
           j = j + 2
-        } else { if nx == "t" {
+          continue
+        }
+        if nx == "t" {
           out = out + "\t"
           j = j + 2
-        } else { if nx == "u" && j + 5 < n {
+          continue
+        }
+        if nx == "u" && j + 5 < n {
           // \uXXXX: MVP - concatenate as-is (6 chars)
           out = out + src.substring(j, j+6)
           j = j + 6
-        } else {
+          continue
+        }
         // Unknown escape: tolerate (keep backslash + char)
         out = out + "\\" + nx
         j = j + 2
-        } } } } } } } } } }
-      } else {
+        continue
+      }
+
       out = out + ch
       j = j + 1
     }
-    }
     // if unterminated, return what we have and the last pos to avoid infinite loops
     return out + "@" + ParserCommonUtilsBox.i2s(j)
   }