💡 alignContent for cross-axis distribution of flex lines

test/align-content.test.ts から始め、vendored されている clay/clay.h のレイアウト構造と、折り返された行を処理するレイアウトパスを調べてください。#42 の文字列リテラルのアライメントに関する作業と、#63 の複数行ラッピングへの依存関係を確認してください。要求された alignContent の各バリアントが、flex-end の例を含め、説明どおりに複数の flex 行を配置し、JS 側のポジショニングを行わない状態になれば完了です。

Is your feature request related to a problem?

Once a container can wrap into multiple flex lines, there's no way to distribute those lines along the cross axis—they pack to the start. With a 2-wide × 6-tall wrap container holding four single-cell children (two lines of two), the lines sit at the top (rows 0-1), with no way to push them to the end, center them, or space them out.

Describe the solution you'd like

An alignContent layout key that distributes free cross-axis space among the flex lines. For the example, alignContent: "flex-end" should push the two lines to the bottom (rows 4-5).

open("root", {
  layout: {
    direction: "ltr",
    flexWrap: "wrap",
    alignContent: "flex-end", // "flex-start" (default) | "flex-end" | "center" | "space-*" | "stretch"
  },
});

alignContent should mirror the CSS align-content property. It's inert until there are multiple flex lines, so it depends on multi-line wrapping (#63).

Describe alternatives you've considered

Without renderer support, the only option is to compute line breaks and cross-axis offsets in JS and emit absolutely-positioned rows—which duplicates the layout engine's line-breaking and can't react to a resize.

Additional context

Failing test case on nm/repro/align-content (test · diff).

Clay's Clay_ChildAlignment (in the vendored clay/clay.h) offers only start/center/end on the cross axis; distributing whole flex lines—including the space-between/around/evenly/stretch variants—is a local addition to the layout pass. New string values should line up with the alignment string-literal refactor in #42 (re: #3).