Add inline-axis edges (inline_start / inline_end) to style spans

[nicoburns]

LLM Contributions: This PR, including this description, was written by Devin (an LLM agent) at the request of @nicoburns. The description would need rewriting by a human before going upstream, per the Linebender LLM policy.

Stacked on linebender#873 (LineMetrics::collapsible_hanging_advance, rebased on upstream main), which is the first commit here and is used to leave collapsible hanging whitespace out of span fragments.

Lets a TreeBuilder style span reserve space before and after its content, which is what CSS inline-axis margin + border + padding on a non-atomic inline box needs. The span stays non-atomic: its text is shaped, wrapped and reordered as before.

API

• TextStyle::inline_start / inline_end and StyleProperty::InlineStart / InlineEnd. Not inherited: push_style_modification_span resets them unless the modification list sets them, and they are ignored on the root style. Only the tree builder supports them.
• Line::span_fragments() yields a SpanFragment (style_index, x, advance, baseline, ascent, descent, has_start_edge, has_end_edge, is_rtl) for every push_style_span span with content or an edge on the line, parents before children. Spans without edges get fragments too, so a consumer can use one path for backgrounds and hit rects.
• Layout::span_edges_mut() to resize edges before (re)breaking lines, analogous to inline_boxes_mut().

Behaviour (matches what Chrome does for box-decoration-break: slice)

• An edge is a LayoutItemKind::SpanStart / SpanEnd item with a fixed advance. Items are only created for spans with a non-zero edge or with no content, so other spans cost nothing in shaping or line breaking.
• A span boundary is not a soft wrap opportunity. The start edge stays on the line of the content after it (a break before that content is taken before the edge), and the end edge on the line of the content before it (if it does not fit, the previous break opportunity is taken). A span with no content stays with the content before it.
• A wrapped span has its start edge only on its first line and its end edge only on its last.
• A non-zero edge is a shaping boundary and makes an otherwise empty line count as having content. Whitespace before a non-zero edge does not hang.
• Edges are included in min/max content widths.
• Collapsible whitespace stays on the side of the boundary it was pushed on.
• Bidi: after the items of a line are reordered, each span's edges are moved to the visual ends of its content on that line, in the paragraph direction (start edge on the left in an LTR paragraph), inner spans first. Text order is never changed by an edge.
• When reordering splits a span into visually discontiguous pieces, it gets one fragment per piece; the edges belong to the outermost pieces.
• Fragments exclude collapsible whitespace hanging at the end of a line, but include preserved whitespace.

Not done

• Spans have no direction of their own: edges are placed in the paragraph direction, and SpanFragment::is_rtl is the paragraph's direction. In CSS the sides depend on the span's own direction.
• box-decoration-break: clone.
• An empty span without edges still gets (zero-advance) edge items so that it has a fragment, which makes it a shaping boundary.
• span_edges_mut() only covers spans built with a non-zero edge or without content.

Tests are in parley_tests/tests/styles/span_edges.rs; they compare against reference layouts rather than snapshots.

Changelog

Added

• TextStyle::inline_start / inline_end (and the matching StyleProperty variants): advances before and after a TreeBuilder style span's content, for CSS inline margin/border/padding. Line::span_fragments returns the per-line extent of each span, and Layout::span_edges_mut allows the edges to be resized after building.
• Breaking change: TextStyle has two new fields.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/38d20c11138046c5a04fb2aa400e7033
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/38d20c11138046c5a04fb2aa400e7033?variant=devin-insiders

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