Support flex-wrap: balance and flex-line-count (CSS Flexbox Level 2)

[nicoburns]

Summary

Adds end-to-end support for CSS Flexbox Level 2 line balancing: flex-wrap: balance (and wrap-reverse balance) and the flex-line-count property, per https://drafts.csswg.org/css-flexbox-2/.

• stylo_taffy::convert::flex_wrap maps Stylo's new FlexWrap::Balance/WrapReverseBalance to taffy::FlexWrap::Balance/BalanceReverse, and TaffyStyloStyle implements FlexboxContainerStyle::flex_line_count() (clamped to >= 1). Gated behind a new on-by-default flexbox_balance cargo feature on stylo_taffy.
• blitz-dom enables the layout.flexbox.balance Stylo pref at document init, so the new syntax parses in Blitz.
• Taffy is now pinned to an upstream main rev (20f04b9f) that includes the merged balancing implementation (Implement flex-wrap: balance (CSS Flexbox Level 2) taffy#1105); Stylo is pinned to the head of the Stylo PR branch (Implement flex-wrap: balance and flex-line-count (CSS Flexbox Level 2) stylo#12, rev 404505d5). The Stylo git rev should be switched to a release once that PR merges and is released.
• Adds focused layout tests (tests/blitz-tests/tests/flex_wrap_balance.rs): four 25px items in a 120px container with 10px gaps split 3/1 with wrap but 2/2 with balance; plus wrap balance, wrap-reverse balance, invalid nowrap balance, and flex-line-count forcing extra lines.

WPT css/css-flexbox/balance: 32/41 run tests pass. The 9 failures are preexisting limitations (7 need script execution which the WPT runner doesn't support, 1 needs width: min-content on atomic inline boxes, 1 needs display: -webkit-box).

Rendered verification (wrap 3/1 vs balance 2/2, wrap-reverse balance, flex-line-count: 2):

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/b7bd5b365d054b87a6053872323832b2
Requested by: @nicoburns

WPT results

28 newly passing, 0 newly failing (net +28).

Full diff (28 changed tests)

+ Fail => Pass css/css-flexbox/balance/balance-001.html
+ Fail => Pass css/css-flexbox/balance/balance-002.html
+ Fail => Pass css/css-flexbox/balance/balance-003.html
+ Fail => Pass css/css-flexbox/balance/balance-005.html
+ Fail => Pass css/css-flexbox/balance/balance-available-size-001.html
+ Fail => Pass css/css-flexbox/balance/balance-available-size-002.html
+ Fail => Pass css/css-flexbox/balance/balance-complex.html
+ Fail => Pass css/css-flexbox/balance/balance-flex-grow.html
+ Fail => Pass css/css-flexbox/balance/balance-line-count-intrinsic-001.html
+ Fail => Pass css/css-flexbox/balance/balance-line-count-intrinsic-003.html
+ Fail => Pass css/css-flexbox/balance/balance-line-count-intrinsic-004.html
+ Fail => Pass css/css-flexbox/balance/balance-line-count-intrinsic-005.html
+ Fail => Pass css/css-flexbox/balance/balance-line-count-intrinsic-006.html
+ Fail => Pass css/css-flexbox/balance/balance-line-count-intrinsic-007.html
+ Fail => Pass css/css-flexbox/balance/balance-min-line-count-001.html
+ Fail => Pass css/css-flexbox/balance/balance-min-line-count-002.html
+ Fail => Pass css/css-flexbox/balance/balance-min-line-count-003.html
+ Fail => Pass css/css-flexbox/balance/balance-min-line-count-004.html
+ Fail => Pass css/css-flexbox/balance/balance-min-line-count-005.html
+ Fail => Pass css/css-flexbox/balance/balance-min-line-count-006.html
+ Fail => Pass css/css-flexbox/balance/balance-negative-margin-001.html
+ Fail => Pass css/css-flexbox/balance/balance-negative-margin-002.html
+ Fail => Pass css/css-flexbox/balance/balance-percentage-size-001.html
+ Fail => Pass css/css-flexbox/balance/balance-percentage-size-002.html
+ Fail => Pass css/css-flexbox/balance/balance-start-bias-001.html
+ Fail => Pass css/css-flexbox/balance/balance-start-bias-002.html
+ Fail => Pass css/css-flexbox/balance/balance-start-bias-003.html
+ Fail => Pass css/css-flexbox/balance/balance-zero-size.html

_{Generated by the WPT workflow.}

[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