Open expressions should accept the leading pipeline marker

[Seddryck]

Problem

The parser rejects an open expression written with the leading pipeline marker:

|> add(1)

This syntax represents an expression whose incoming value is supplied by its evaluation context, with add(1) as its first pipeline stage.

Expected behavior

|> add(1) parses successfully as an open expression.

Actual behavior

The grammar requires an expression before any pipeline separator, so the leading |> is reported as an error.

Acceptance criteria

• |> add(1) parses as an open expression without error.
• Existing closed and open expression syntax continues to parse.
• Parser corpus and language-binding tests pass.