Clarify the equivalence of actions in default actions

[shyukahn]

Closes #1388
This PR is made from the April 2026 P4 LDWG meeting.

A default_action of a table must be one of the actions that appear in the actions list. The PR clarifies how to determine the equivalence of the default action and an action in the actions list.

• The name of the default action must be same as the action of the actions list, after resolving them both to full hierarchical names.
• The expressions passed as in, out, or inout parameters must be syntactically identical to the expressions used in the action of the actions list. A P4 implementation may choose to accept default actions with arguments that are semantically equivalent to the expressions in the actions list, instead of requiring them to be syntacitcally identical.

[jafingerhut]

LGTM