Add pure Rust implementations of the SHA-2 family per FIPS 180-4
in the crypto crate:

- SHA-256: 256-bit digest, 64-byte blocks, 64 rounds
- SHA-512: 512-bit digest, 128-byte blocks, 80 rounds
- SHA-384: truncated SHA-512 with different initial values

All three provide streaming (new/update/finalize) and one-shot APIs.
21 tests including NIST test vectors (empty, abc, 448-bit, 896-bit,
million-a) plus streaming and edge-case coverage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

When both underline and background were present, bg_idx pointed to
the underline rect instead of the DrawGlyphs command, causing the
background to paint after the text. Capture glyph_idx before pushing
any commands to ensure correct painter's order.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Replace the simplified inline text layout with a proper inline
formatting context (IFC):

Layout crate:
- Flatten inline tree into items (words, spaces, breaks, inline starts/ends)
- Word-wrap into line boxes respecting per-fragment font sizes
- Handle <br> as forced line breaks
- Apply text-align (left, center, right) via horizontal offset
- Use computed line-height instead of hardcoded font_size * 1.2
- Inline box model: margin/padding/border offsets for inline elements
- TextLine now carries per-fragment styling (font_size, color,
text_decoration, background_color) instead of inheriting from parent
- Store text_align and line_height on LayoutBox

Render crate:
- paint_text uses per-TextLine styling (color, font_size, text_decoration)
- Render inline fragment backgrounds when not transparent
- Underline uses per-fragment font_size for baseline calculation

Tests: 7 new (per-fragment styling, <br>, text-align center/right,
inline padding, font_size per fragment, line-height)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>