Fix whitespace collapsing removing spaces at style span boundaries

[nicoburns]

LLM Contributions: This PR (code and description) was produced by Devin, requested and reviewed by @nicoburns.

TreeStyleBuilder in WhiteSpaceCollapse::Collapse mode was trimming whitespace at every style span boundary:

• push_style_span set is_span_first = true, so the first text of every span had its leading whitespace stripped even when the preceding text ended in a non-space character.
• pop_style_span committed with is_span_last = true, stripping the span's trailing whitespace unconditionally.

So "Singer-songwriter" + <span>"\u{a0} · "</span> + "actress" collapsed to Singer-songwriter·actress — dropped separators and no soft-break opportunities. In Blitz this breaks Wikipedia infoboxes (hlists rendered as display: inline <li>s with ::after separators): the unbreakable text inflates the intrinsic width, overflowing the cell and misplacing the table columns (see https://en.wikipedia.org/wiki/Dolly_Parton, the about=#mwt9 infobox).

Fix: leading trim now only happens at the start of the whole text or after existing trailing whitespace (is_span_first is no longer reset per-span), and trailing trim only happens at the end of the whole layout (pop_style_span commits with is_span_last = false; finish still trims the end).

Known limitation: if the layout's final text ends inside a span with trailing collapsible whitespace, that single trailing space is now kept (previously the per-span trim removed it).

Blitz rendering of the Dolly Parton infobox with this fix (previously the text overflowed the infobox and the columns were misplaced):

Changelog

Fixed

• Whitespace collapsing in TreeBuilder no longer removes spaces at style span boundaries.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/dabfa60a5ab3411bb6c6225f9f81a79e
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/dabfa60a5ab3411bb6c6225f9f81a79e?variant=devin-insiders

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring