Fix inline vertical-align top and bottom alignment

[YOSEFTT]

Fix inline vertical-align top and bottom alignment

Summary

Fixes reversed placeholder alignment for inline vertical-align: top and
vertical-align: bottom.

The previous placeholder mapping made top and bottom render in the opposite
positions. sub and super still appeared correct because their padded wrapper
adds a second alignment adjustment, so the two effects canceled each other out.
Plain top and bottom do not use that wrapper, which exposed the reversed
mapping.

Changes

• Map vertical-align: top to PlaceholderAlignment.bottom.
• Map vertical-align: bottom to PlaceholderAlignment.top.
• Keep the existing sub and super placeholder mappings so their padded
  alignment behavior stays unchanged.
• Update regression expectations for inline text, styled text, inline elements,
  inline-block elements, and images.

Verification

• git diff --check
• flutter test test/style_vertical_align_test.dart from packages/core
  • 00:53 +26: All tests passed!

Note: this is a Flutter widget test, so the supported verification command is
flutter test. A direct dart test invocation is not appropriate here; it
loads the test on the plain Dart VM and fails because Flutter's dart:ui
library is unavailable there.

[YOSEFTT]

verified in a production Flutter app,
This is the code I wrote in the file:

Normal text <span style="vertical-align:super;">Raised to full height</span> Normal text <span style="vertical-align:sub;">lowered to full height</span> <span style="vertical-align:top;">Relegated to the bottom of the line</span> Normal text <span style="vertical-align:bottom;">Raised to the top of the line</span>

This is how it actually looks in the app:

If you want, I can also add a test that measures real position in the papillae to prove it objectively.