Add built-in CellAlignments presets for grid cell content positioning

[nicolasguelfi]

Description

st_grid cells frequently need content alignment (horizontal, vertical, or both), yet the library provides no ready-made styles for this common use case. Users must manually compose flexbox CSS strings every time:

cell_center = Style(
    "display:flex;flex-direction:column;"
    "justify-content:center;align-items:center;text-align:center;",
    "cell_center",
)

The existing building blocks (Flex, Layouts, Alignments) are close but insufficient:

• Layouts.vertical_center_layout uses display:flex without flex-direction:column, so vertical centering doesn't work in grid cells (which stack content vertically).
• Flex.center_justify / Flex.center_align_items are individual properties, not composable cell presets.
• Alignments.center_align only sets text-align — no flexbox positioning.

Motivation

Grid cells are the most common context where both horizontal and vertical alignment matter. Every presentation and multi-column layout needs this. Currently, every project must define its own cell alignment styles, leading to duplicated boilerplate across all StreamTeX projects.

Proposed Solution

Add a CellAlignments class in streamtex/styles/container.py, following the same pattern as Flex, Layouts, Positions, etc.

Implementation

class CellAlignments:
    """Content alignment presets for st_grid cells.

    Each preset uses ``display:flex;flex-direction:column`` as its base,
    which aligns with ``GridController.cell()``'s vertical stacking model.
    The ``text-align`` property is included where appropriate so that
    inline text content is also aligned consistently.

    Naming convention: ``<horizontal>_<vertical>``
    - horizontal: left, center, right
    - vertical: top, middle, bottom
    """

    # ── Centered horizontally ──────────────────────────────────────
    center_top = Style(
        "display:flex;flex-direction:column;"
        "justify-content:flex-start;align-items:center;text-align:center;",
        "cell_center_top",
    )
    center_middle = Style(
        "display:flex;flex-direction:column;"
        "justify-content:center;align-items:center;text-align:center;",
        "cell_center_middle",
    )
    center_bottom = Style(
        "display:flex;flex-direction:column;"
        "justify-content:flex-end;align-items:center;text-align:center;",
        "cell_center_bottom",
    )

    # ── Left-aligned horizontally ──────────────────────────────────
    left_top = Style(
        "display:flex;flex-direction:column;"
        "justify-content:flex-start;align-items:flex-start;",
        "cell_left_top",
    )
    left_middle = Style(
        "display:flex;flex-direction:column;"
        "justify-content:center;align-items:flex-start;",
        "cell_left_middle",
    )
    left_bottom = Style(
        "display:flex;flex-direction:column;"
        "justify-content:flex-end;align-items:flex-start;",
        "cell_left_bottom",
    )

    # ── Right-aligned horizontally ─────────────────────────────────
    right_top = Style(
        "display:flex;flex-direction:column;"
        "justify-content:flex-start;align-items:flex-end;text-align:right;",
        "cell_right_top",
    )
    right_middle = Style(
        "display:flex;flex-direction:column;"
        "justify-content:center;align-items:flex-end;text-align:right;",
        "cell_right_middle",
    )
    right_bottom = Style(
        "display:flex;flex-direction:column;"
        "justify-content:flex-end;align-items:flex-end;text-align:right;",
        "cell_right_bottom",
    )

    # ── Convenience alias (most common case) ───────────────────────
    center = center_middle

Integration points

1. container.py — add the class after GridStyles
2. Container facade — add cells = CellAlignments (line ~398)
3. __init__.py — export CellAlignments in the container imports
4. StxStyles shortcut (optional) — add cell_center = Container.cells.center alongside center_txt

Access patterns

# Via Container facade
Container.cells.center_middle
Container.cells.left_top

# Via StxStyles shortcut (if added)
s.cell_center

# Composable with other styles
Container.cells.center_middle + Paddings.medium_padding + BackgroundColors.azure_bg

Naming rationale

• CellAlignments (not CellStyles) — mirrors Alignments for text, scoped to cell context
• <h>_<v> naming — reads naturally: "center middle", "left top", "right bottom"
• center alias → center_middle — the most frequent use case gets the shortest name

Environment

Key	Value
StreamTeX	0.6.9
Python	3.13.7
OS	Darwin 25.3.0 arm64
UV	0.10.2
Branch	main
Commit	f1a4c79