Add TreeBuilder::text and has_pending_whitespace; process tree builder text eagerly

[nicoburns]

LLM Contributions: Code and this description were written by Devin (an LLM agent). The description needs rewriting by a human before this goes upstream.

Adds two getters to TreeBuilder:

• text(&self) -> &str returns the text pushed so far, after white space processing. Trailing collapsible whitespace is not included.
• has_pending_whitespace(&self) -> bool reports whether there is trailing collapsible whitespace. It becomes a single space if more content follows, or is removed otherwise.

Blitz needs these for text-transform: capitalize, which has to find word boundaries in the text before the current text node (DioxusLabs/blitz#947).

To keep the getters read-only, TreeStyleBuilder no longer buffers text in uncommitted_text, and push_text now applies white space processing straight away. The buffer was needed when collapsing trimmed whitespace at each span's start and end, which required all of a span's text before processing. Since linebender#786 that is handled by the streaming pending_whitespace state. splitting_text_does_not_affect_white_space_processing checks that splitting text across push_text calls at any char boundary gives the same text and style runs, in all four WhiteSpaceCollapse modes.

Performance was measured with TreeBuilder benchmarks that are kept separately in the draft PR stacked on this one. Tango comparison against the buffered version:

• push, 10 chars per push_text (1 or 8 pushes per span): 0% to 41% faster. For example, latin 8000 chars with Preserve and 1 push per span went from 33.5 µs to 24.5 µs. The buffered version allocated a new buffer for every span.
• push, 1 char per push_text with 10 pushes per span, the worst case: up to 31% slower, about 2–4 ns more per call.
• build, including shaping: between −3% and +1.2%.
• Blitz laying out the Barack Obama Wikipedia page: the text-push step was 1.2% faster, and the change in total layout time was within noise.

Changelog

Added

• TreeBuilder::text and TreeBuilder::has_pending_whitespace, which return the white space processed text pushed so far and whether collapsible whitespace is pending after it.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/4641a2bf5eca4d4fb140d16886180423
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/4641a2bf5eca4d4fb140d16886180423?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring