feat: add underline and strikethrough decoration options

[rsubrama83]

Closes #11

What

New text style methods on TextStyle and span descriptors:

Area	Methods
Line style	DecorationSolid() (default), DecorationDouble(), DecorationDotted(), DecorationDashed(), DecorationWavy()
Color	DecorationColor(string), independent of the text color
Thickness	DecorationThickness(float), in points
Underline position (sub/superscript)	UnderlinePositionAuto() (default), UnderlineAtBaseline(), UnderlineBelowGlyphs()

Line style, color and thickness apply to underline and strikethrough (like CSS text-decoration-*). All properties take part in normal style inheritance.

text.Span("typo").Underline().DecorationWavy().DecorationColor(Colors.Red.Medium);

text.DefaultTextStyle(x => x.Underline().UnderlineAtBaseline());
text.Span("H");
text.Span("2").Subscript();   // underline now in line with "H" and "O"
text.Span("O");

Compatibility

• Defaults (Solid, Auto, text color, font thickness) reproduce the current output exactly; solid lines still go through the existing rectangle drawing.
• ICanvas (internal) gets DrawTextDecoration for dotted, dashed and wavy lines (Skia path effects / paths). No public breaking change.

Verification

• Release build green, 184 unit tests pass (16 new in TextDecorationTests: placement for each position mode, color/thickness/style for underline and strikethrough, inheritance, validation, full PDF generation).
• New examples TextExamples.DecorationStyles and TextExamples.SuperscriptSubscript_UnderlinePosition rendered and checked visually.

Docs

docs/reference/text.md: new subsections for decoration line options and underline position, updated style table and library defaults. AGENTS.md current focus updated.

🤖 Generated with Claude Code

[1fabi0]

I like this change but for strikthrough such options as Baseline or Glyph level would also be very nice currently only this strikthrough is possible:

[rsubrama83]

Added in 826b769: StrikethroughAtBaseline() draws the strikethrough at normal-text height and thickness so it runs continuously across sub/superscripts (e.g. E=mc²). StrikethroughThroughGlyphs() stays the default, so existing output is unchanged.