[css-align-3] Abspos alignment in vertical axis should care about align-self, just like horizontal/justify-self

When an abspos element has left:auto and right:auto, the sizing/positioning defined in CSS2 is partially dependent on 'direction'. In Align, we generalized that to instead depend on justify-self; this automatically respects 'direction', but allows alignment on either side without screwing with your text direction, and allows more powerful things too, such as center alignment. (Defined in section 6.5.)

CSS2 didn't have any special rules for vertical alignment, tho: if they're both auto, you just treat bottom as 0 and then solve normally. So, we didn't follow up and apply the same "upgrade" to abspos+align-self.

That seems inconsistent and bad; the self-alignment properties apply in whatever axis the item is "alone" in its container, and abspos elements are alone in both axises. If you set place-self: center, it would be annoying and weird for it to center horizontally, but not vertically.

So, the 6.5 rules for sizing/aligning should similarly apply in the vertical axis for align-self.

Start with CSS Align section 6.5 and the CSS2 absolutely positioned sizing and positioning rules described in the issue. Trace how the horizontal justify-self treatment maps to the vertical axis, then update the specification so align-self governs the corresponding auto-inset cases and confirm the wording consistently covers place-self.