[css-contain-3] Clarify Container Relative Lengths

The spec says:

unit	relative to
cqi	1% of a query container’s inline size
cqb	1% of a query container’s block size

...

For each element, container query length units are evaluated as container size queries on the relevant axis (or axes) described by the unit.

However, according to #6873, the query container axis that will be queried should depend on the writing-mode of the element. In other words, cqi could be relative to the query container's inline size or its block size, depending on which axis the element specifies to be the inline axis.

As written, the spec seems to imply that e.g. cqi should query the query container's inline size regardless of the element using the unit, which is inconsistent with how other units like svi and svb work.

Start with the CSS Containment Level 3 specification linked in the issue, then compare the container query length wording with the behavior discussed in issue #6873 and with the svi/svb definitions. Clarify whether cqi and cqb use the querying element’s writing mode, and update the specification text so the intended axis behavior is unambiguous.