Fix None filtering, scope serialization, and Assignment RHS scoping

[Copilot]

Three correctness issues identified in code review of the lexical scope feature.

Changes

• builder.py — visit_module_definition now skips None items when flattening children (via elif item is not None). visit_statement() can return None for empty nodes; those were propagating into ModuleDeclaration.children and crashing build_scope().

• serialization.py — scope is now excluded from field serialization alongside position. After build_scopes() runs, node.scope is a Scope instance that _serialize_value() can't handle, causing TypeError in ast_to_dict/json/yaml.

• nodes.py — Assignment.build_scope() now always passes parent_scope (the full scope) to the RHS expression. Function literal bodies are closures that resolve variables lazily at call time, so the full enclosing scope — including the variable being assigned — must be visible. This correctly handles both direct function literal assignments and expressions that contain function literals (e.g. a ternary whose branches are function literals):

fn = function(n) n == 0 ? 1 : fn(n-1);  // fn IS visible in body for recursion
a = b ? function(x, n) a(x+n, n-1) : function(x, n) a(x*n, n-1);  // a IS visible in both branches

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