DAL_node: propagate shards one level earlier

What

Closes #6899 (closed).

Changes the stream in the DAL node’s crawler to one that yields the predecessor level of each freshly observed head (i.e. HEAD~1), together with the finalized heads (HEAD~2) stream, providing the building block for starting shard propagation one level earlier (published_level +1) as part of the “Reduced DAL latency” plan.

Why

How

Manually testing the MR

• tezt-cloud journal of experiments (redone after design change)

• CI green (especially the dal.ml tezt test)

• newly added tezt test:

dune exec tezt/tests/main.exe -- --file dal.ml --title 'Alpha: Testing DAL node (publish slot in one level reorganisation)'

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR