feat(text): verticalAlign works without maxHeight (flex-friendly)

[chiefcll]

Summary

Builds on #28. After that PR, verticalAlign activated whenever maxHeight > 0. This change goes further: verticalAlign now activates whenever the containing box is taller than the intrinsic text height — even without maxHeight. The "containing box" is maxHeight if set, otherwise the node's own h (which a flex parent or the user may have grown).

This makes verticalAlign Just Work inside flex layouts:

<View style={{ display: 'flex', alignItems: 'flexStart', height: 300, ... }}>
  <Text style={{ fontSize: 44, verticalAlign: 'middle' }}>A</Text>
  <Text style={{ fontSize: 80, verticalAlign: 'middle' }}>B</Text>
</View>

Each text's caps land dead-center inside the 300px row, with no maxHeight boilerplate.

Implementation

src/core/CoreTextNode.ts:152-161 — switched the slack formula from maxHeight - h to (maxHeight ?? h) - intrinsicH, where intrinsicH = this._renderInfo.height:

const intrinsicH = this._renderInfo.height;
const boxH = hasMaxHeight === true ? maxHeight : h;
const slackY = boxH - intrinsicH;
if (slackY > 0) {
  if (verticalAlign === 'bottom')      containY = slackY;
  else if (verticalAlign === 'middle') containY = slackY * 0.5;
}

This also fixes a subtle bug in the original formula: maxHeight - h was wrong when h had been modified post-load (e.g., by flex), because h no longer equaled the intrinsic text height. The new formula uses intrinsicH explicitly.

Other changes

• src/core/text-rendering/TextRenderer.ts — verticalAlign docstring updated to describe the new activation rule.
• examples/tests/text-vertical-align.ts — added a 4th VRT row ("Explicit h, no maxHeight") that creates a text node, awaits the loaded event, then sets node.h = 200 to exercise the new code path.

Test plan

• pnpm build clean.
• vitest run — 182/182 pass.
• Recapture VRT (text-vertical-align-*.png) — will be done before merge. Only the text-vertical-align snapshots are expected to change (added one row); other tests don't grow h past intrinsic without also setting maxHeight.

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