Implement text-transform: capitalize; make uppercase/lowercase language-aware

[nicoburns]

Summary

Adds text-transform: capitalize. uppercase and lowercase are now language-aware: non-ASCII text (and Turkic-language text) is mapped with ICU4X instead of str::to_uppercase/to_lowercase, and ASCII text takes a byte-mapping fast path that gives the same output. The logic is in a new layout/text_transform.rs, called from build_inline_layout_recursive.

Uses linebender/parley#856 (TreeBuilder::text and TreeBuilder::has_pending_whitespace). parley is pinned to linebender main at 62790943, the #856 merge. That rev changed Run::font() to return &FontData, so blitz-paint/src/text.rs is updated to match.

• Words can span text nodes: hel<b>lo</b> → "Hello", and in a<span capitalize>b</span>c the "b" stays lowercase. A capitalize text node looks back at the last 32 bytes of builder.text() (the white-space-processed text pushed so far), starting from the last forced word break. If builder.has_pending_whitespace(), the node starts a new word, so no context is used. Nothing is copied or tracked for text nodes that aren't capitalized.
  • Only look-behind is needed: under UAX#29, whether there is a word break before a letter never depends on the text after it.
  • The earlier text in the context has already been case-transformed. That's fine for segmentation, since case mapping keeps letters as letters.
• TextTransformer stores context_start, the builder text length at the last forced word break, plus reusable scratch/output buffers. word_break(builder) is called after a position-inside list marker, at in-flow inline boxes (replaced elements, inline-block, etc.) and at <br>. Out-of-flow and float boxes add no text and don't break words (p<span abs></span>ass → "Pass").
• transform returns &str. Unchanged text is returned borrowed, with no allocation or copy (OutputSink only starts writing to the reused buffer at the first difference). ASCII text takes a byte-mapping fast path for uppercase/lowercase, except for Turkic languages (ascii_fast_path_matches_icu checks it against ICU4X).
• capitalize runs icu_segmenter word segmentation over context + text. In each word that starts inside the current node, the first typographic letter unit (general category Letter or Number) is titlecased with icu_casemap's TitlecaseMapper (TrailingCase::Unchanged). This skips leading punctuation and leaves circled letters such as ⓐ alone.
• The language comes from the computed -x-lang (the lang attribute). This enables Dutch ij→IJ and Turkish/Azeri dotted and dotless i. casing_language drops the language when an explicit script subtag contradicts it (tr-Cyrl → root casing), as css-text-3 § script tagging requires. ICU4X itself only looks at the language subtag.
• New blitz-dom feature icu-text-transform (on by default) gates icu_casemap and writeable. It's forwarded and on by default in blitz, dioxus-native and dioxus-native-dom, and enabled explicitly in apps/browser and the WPT runner. Without it, uppercase/lowercase/titlecase_segment fall back to std char::to_uppercase/to_lowercase (final sigma via str::to_lowercase). Word segmentation and the letter check are the same either way, but casing is language-insensitive and titlecase is approximated by uppercase (dž→DŽ, ß→SS). CI's Clippy job also lints blitz-dom with the feature off.
• New dependencies: icu_casemap, icu_casemap_data, icu_locale_core, icu_properties and icu_segmenter, all at 2.3, plus writeable 0.6. Only icu_casemap and icu_casemap_data are new to the tree. Release blitz browser binary (production profile): +71 KiB (+0.26%) with the feature on, about 42 KiB of case-mapping data and 27 KiB of code.

WPT css/css-text/text-transform: 57/113 → 92/113, no regressions. Remaining capitalize failures:

• capitalize-033: a pixel difference that still happens with text-transform removed and "Pass" typed literally. The likely cause is the inline box splitting the shaping run; not investigated further.
• capitalize-036: needs innerText.

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

WPT results

Subtests: 53 newly passing, 0 newly failing (net +53).

Full diff (53 changed tests)

+ FAIL => PASS  [1/1]  +1  css/CSS2/css1/c545-txttrans-000.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-001.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-001.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-002.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-003.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-005.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-006.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-007.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-008.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-009.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-010.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-011.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-applies-to-014.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-bicameral-005.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-bicameral-006.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-cap-001.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-cap-002.xht
+ FAIL => PASS  [1/1]  +1  css/CSS2/text/text-transform-cap-003.xht
+ FAIL => PASS  [1/1]  +1  css/css-display/run-in/text-transform-applies-to-004.xht
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-001.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-003.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-005.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-007.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-009.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-010.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-014.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-016.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-018.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-020.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-022.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-024.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-028.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-030.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-031.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-032.xht
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-034.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-capitalize-035.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-letter-spacing-002.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-001.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-001a.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-002.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-002a.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-003.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-004.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-005.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-tailoring-document-lang-dynamic.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-upperlower-039.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-upperlower-040.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-upperlower-041.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-upperlower-042.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-upperlower-043.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-upperlower-044.html
+ FAIL => PASS  [1/1]  +1  css/css-text/text-transform/text-transform-upperlower-106.html

_{Generated by the WPT workflow.}

[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