Decide whether a shape binding takes Write or Control

[tophcodes]

What

Changing a container's ldp:constrainedBy binding is an ordinary write to the container's own graph, so it takes acl:Write on the container. Decide whether that is the right bar or whether it should take acl:Control.

Why

The binding is not content, it is policy about content: it decides what every future write into that container must satisfy. An agent granted Write on a container to put documents in it can also remove the constraint those documents were supposed to meet, and nothing in the response says a constraint was dropped.

acl:Control is the mode that already means "may change the rules rather than the data".
Whether shape validation counts as rules is the actual question: it is arguable that a shape
is part of what a container is, and that Write is correct.

How

• Settle it as a decision, not an implementation detail; docs/decisions.md if the answer is anything other than "Write, obviously".
• If it becomes Control, the check belongs beside the shape lookup on the write path (Fold the shape binding lookup into a store call the write path already makes #7), not as a second pass.
• Either way, say it in docs/uri-space.md next to the auxiliary table, which already states the authorizing mode for every other kind.