Refactor Binder to use Diagnostics

The Binder and its helper functions have been updated to accept and
propagate a `Diagnostics` struct. This allows for better error reporting
during the binding phase, enabling the compiler to continue processing
even after encountering errors and collect all issues before halting.

The `BoundKind::Error` node and `StaticType::Error` are introduced as
"poison" nodes/types. These nodes indicate that an error occurred during
compilation, preventing further valid processing of that specific AST
fragment but allowing the compiler to continue with other parts of the
code.

The `Parser` has also been updated to return a `Diagnostics` struct,
enabling it to report errors during the initial parsing stage while
still attempting to build a partial AST. This adheres to the same error
recovery strategy.

src/ast/types.rs

@@ -306,6 +306,8 @@ pub enum StaticType {
     Function(Box<Signature>),
     FunctionOverloads(Vec<Signature>),
     Object(&'static str),
+    /// A diagnostic poison type, allowing type-checking to continue after an error.
+    Error,
 }
 
 impl fmt::Display for StaticType {
@@ -361,6 +363,7 @@ impl fmt::Display for StaticType {
                 write!(f, "overloads({} variants)", sigs.len())
             }
             StaticType::Object(name) => write!(f, "{}", name),
+            StaticType::Error => write!(f, "<error>"),
         }
     }
 }
@@ -368,7 +371,7 @@ impl fmt::Display for StaticType {
 impl StaticType {
     /// Returns true if `other` can be assigned to a location of type `self`.
     pub fn is_assignable_from(&self, other: &StaticType) -> bool {
-        if self == other || matches!(self, StaticType::Any) || matches!(other, StaticType::Any) {
+        if self == other || matches!(self, StaticType::Any) || matches!(other, StaticType::Any) || matches!(self, StaticType::Error) || matches!(other, StaticType::Error) {
             return true;
         }