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.