fix: Phase 3 — #25,28,29,41 token/AST/parser cleanup

Files modified (4):
  compiler/token/token.go — #25: Replace hand-rolled itoa with strconv.Itoa
    Fixes math.MinInt overflow bug in original implementation
  compiler/ast/ast.go — #29: Fix VarDecl.End() returning last var position
    Was returning Pos() (useless span info)
  compiler/parser/stmtreg.go — #28: Eliminate all 7 token array mutations
    rewriteAsIdent() modifies p.current only, not the token array
    Prevents backtracking corruption and improves safety
  compiler/lexer/lexer.go — Already clean from Phase 2

Issues resolved: #25 (MEDIUM), #28 (MEDIUM), #29 (MEDIUM), #41 partial (LOW)
Total fixed: 29/53

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

compiler/token/token.go

@@ -6,6 +6,8 @@
 // (ref/typescript-go/internal/ast/kind.go, precedence.go).
 package token
 
+import "strconv"
+
 // Kind represents a token type. Using int16 following tsgo pattern.
 type Kind int16
 
@@ -215,33 +217,12 @@ type Position struct {
 
 func (p Position) String() string {
 	if p.File != "" {
-		return p.File + ":" + itoa(p.Line) + ":" + itoa(p.Col)
+		return p.File + ":" + strconv.Itoa(p.Line) + ":" + strconv.Itoa(p.Col)
 	}
-	return itoa(p.Line) + ":" + itoa(p.Col)
+	return strconv.Itoa(p.Line) + ":" + strconv.Itoa(p.Col)
 }
 
-// simple int-to-string without importing strconv
-func itoa(n int) string {
-	if n == 0 {
-		return "0"
-	}
-	buf := [20]byte{}
-	i := len(buf) - 1
-	neg := n < 0
-	if neg {
-		n = -n
-	}
-	for n > 0 {
-		buf[i] = byte('0' + n%10)
-		i--
-		n /= 10
-	}
-	if neg {
-		buf[i] = '-'
-		i--
-	}
-	return string(buf[i+1:])
-}
+// itoa removed — using strconv.Itoa (fixes math.MinInt overflow bug)
 
 // --- Operator Precedence (tsgo pattern) ---