feat: Harbour RTL vs Go math comparison example + analyzer IMPORT fix

- examples/go_math_compare.prg: Side-by-side comparison of
  Harbour RTL (Abs, Sqrt, Round, Int, Max, Min, Log, Exp, Mod)
  vs Go math package (Sin, Cos, Pow, Pi, Floor, Ceil, Hypot, ...)
- Combined usage: normal distribution, compound interest, distance
- Analyzer: recognize IMPORT package names as valid identifiers
- Analyzer: add math RTL functions (ABS, SQRT, etc.) to known list

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

compiler/analyzer/analyzer.go

@@ -369,6 +369,15 @@ func (a *Analyzer) checkVarUsage(name string, pos token.Position) {
 		return
 	}
 
+	// Check if it's an IMPORT package name
+	for _, imp := range a.file.Imports {
+		parts := strings.Split(imp.Path, "/")
+		pkgName := parts[len(parts)-1]
+		if strings.EqualFold(pkgName, name) || (imp.Alias != "" && imp.Alias == name) {
+			return
+		}
+	}
+
 	// Not declared — warn (could be MEMVAR, FIELD, or typo)
 	a.warn(pos, "undeclared variable '%s' (missing LOCAL?)", name)
 }
@@ -407,6 +416,8 @@ func (a *Analyzer) isKnownFunction(name string) bool {
 		"SLEEP": true, "HB_IDLEADD": true, "SECONDS": true,
 		"ERRORBLOCK": true, "BREAK": true, "PCOUNT": true, "PROCNAME": true,
 		"SETPOS": true, "ROW": true, "COL": true, "MAXROW": true, "MAXCOL": true,
+		"ABS": true, "INT": true, "ROUND": true, "SQRT": true, "LOG": true, "EXP": true,
+		"MAX": true, "MIN": true, "MOD": true,
 		"SETCOLOR": true, "DISPBOX": true, "DISPBEGIN": true, "DISPEND": true,
 		"HB_SYMBOL_UNUSED": true, "HB_DEFAULT": true, "HB_NTOS": true,
 	}