Add new TexBlock and Hit testing APIs

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This PR expands the text layout stack into a richer prepared-layout and interaction API surface.

Highlights:

• Adds prepared TextBlock layout APIs for repeated measuring, rendering, per-line inspection, and manual flow.
• Adds LineLayout and LineLayoutEnumerator so callers can inspect, render, and position individual laid-out lines.
• Adds text interaction APIs for hit testing, caret positioning, caret movement, word selection, and selection bounds.
• Adds bidi-aware caret/selection behavior, including split visual selection rectangles for mixed-direction text.
• Adds TextBidiMode to support normal bidi resolution and explicit direction override behavior.
• Adds paragraph/editor interaction modes so paragraph layout trims trailing whitespace while editor layout preserves typed trailing spaces and terminal hard breaks.
• Adds soft-hyphen rendering support with TextHyphenation and custom hyphen markers.
• Adds max-line ellipsis support with TextEllipsis and custom ellipsis markers.
• Adds inline placeholder support using object replacement semantics, placeholder metrics, alignment, and line-height participation.
• Consolidates glyph detail APIs into GetGlyphMetrics, exposing advance, bounds, renderable bounds, code point, and source mapping from one glyph metrics entry.
• Updates the design documentation for the new measurement, layout, and interaction APIs.

Breaking API Changes

• Replaces the old separate glyph measurement APIs with a single GetGlyphMetrics API.
• Renames the font-table glyph metrics type to FontGlyphMetrics.
• Uses GlyphMetrics for laid-out glyph entries exposed by text measurement APIs.

Testing

This includes unit and visual coverage for:

• Soft hyphen behavior, including none/custom modes, fallback runs, bidi text, and visual output.
• Ellipsis behavior, including standard/custom markers, max-line truncation, and visual output.
• Bidi mode behavior, including normal vs override ordering.
• Caret placement and movement, including bidi cases, start/end placement, line movement, and preserved line navigation position.
• Selection geometry, including bidi-discontinuous selection ranges.
• Paragraph vs editor whitespace handling.
• Placeholder layout, alignment, line-height effects, source mapping, and visual output.
• Per-line layout enumeration and manual flow rendering.

[codecov]

Codecov Report

❌ Patch coverage is 89.10035% with 315 lines in your changes missing coverage. Please review.
✅ Project coverage is 84%. Comparing base (bf9f4aa) to head (98c6466).

Files with missing lines	Patch %	Lines
src/SixLabors.Fonts/TextInteraction.cs	74%	68 Missing and 50 partials ⚠️
src/SixLabors.Fonts/FontGlyphMetrics.cs	75%	39 Missing and 10 partials ⚠️
src/SixLabors.Fonts/TextLayout.LineBreaking.cs	89%	19 Missing and 11 partials ⚠️
src/SixLabors.Fonts/TextLine.cs	93%	15 Missing and 13 partials ⚠️
src/SixLabors.Fonts/LineLayout.cs	76%	20 Missing ⚠️
src/SixLabors.Fonts/TextBlock.Visitors.cs	95%	7 Missing and 5 partials ⚠️
src/SixLabors.Fonts/TextLayout.cs	95%	3 Missing and 5 partials ⚠️
...es/AdvancedTypographic/GPos/LookupType3SubTable.cs	12%	7 Missing ⚠️
src/SixLabors.Fonts/TextMeasurer.cs	75%	6 Missing and 1 partial ⚠️
src/SixLabors.Fonts/PlaceholderGlyphMetrics.cs	90%	6 Missing ⚠️
... and 16 more

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #530     +/-   ##
=======================================
  Coverage     83%     84%             
=======================================
  Files        312     332     +20     
  Lines      22737   24634   +1897     
  Branches    3349    3609    +260     
=======================================
+ Hits       19018   20737   +1719     
- Misses      2905    3010    +105     
- Partials     814     887     +73     

Flag	Coverage Δ
unittests	84% <89%> (+<1%)	⬆️

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