expr.go: add exprUintptr support in assignmentExpression for union pointer access

The expression pattern (result = fn()).ptr where the result is a tagged union (Subtree = union of inline data and pointer) failed because assignmentExpression only handled exprDefault/exprSelect/exprVoid modes, not exprUintptr.

When mode is exprUintptr, execute the assignment and return the address of the LHS as uintptr, matching the pattern used by primaryExpression.

Fixes ccgo compilation of tree-sitter parser.c:762: while ((result = reusable_node_tree(&self->reusable_node)).ptr) {

Co-Authored-By: DeepSeek V4 Pro (1M context) zhangheng@dscli.io