feat: support top / center / bottom vertical alignment within containers

[teezeit]

Summary

Similar to how horizontal position modifiers (left, center, right) are supported for aligning content within a container, we should support vertical equivalents: top, center, bottom.

Use case

A layout container should be able to pin content to the top, middle, or bottom of its vertical axis — useful for card grids, hero sections, side-by-side columns, and any container where content height varies across cells.

Proposed syntax

Mirror the existing horizontal modifier pattern:

::: grid-2 card {.align-top}

### Short title
Brief text.

### Longer title
This card has more content and would otherwise push alignment off.

:::

Or as an inline modifier on individual children:

::: row
Content pinned to top. {.top}

Content pinned to bottom. {.bottom}
:::

Expected behaviour

Modifier	Effect
top	Aligns content to the top of the container / cell
center (vertical)	Centers content vertically
bottom	Aligns content to the bottom of the container / cell

Implementation notes

• AST: extend the existing modifier/class system on container nodes and/or child nodes
• Renderer: map to align-items: flex-start / center / flex-end (or justify-content depending on axis) in the generated HTML
• Should compose cleanly with existing horizontal alignment modifiers