Implement flex-wrap: balance and flex-line-count (CSS Flexbox Level 2, Servo-only)

[nicoburns]

Summary

Port of DioxusLabs/stylo#12 to the Firefox tree (servo/components/style). Servo-engine-only, gated behind layout.flexbox.balance (always false on Gecko builds):

• flex-wrap switches from a generated keyword property to a custom specified::FlexWrap type (also the computed value) parsing the Level 2 grammar nowrap | [ wrap | wrap-reverse ] || balance:

  pub enum FlexWrap { Nowrap, Wrap, WrapReverse, Balance, WrapReverseBalance }

  wrap balance / balance wrap / bare balance → Balance; wrap-reverse balance → WrapReverseBalance; nowrap balance rejected. Serializes to canonical balance / wrap-reverse balance. The Balance* variants are #[cfg(feature = "servo")], and the balance keyword only parses when static_prefs::pref!("layout.flexbox.balance") is true (that pref lookup is also servo-cfg'd, so Gecko never references a nonexistent pref).
• New flex-line-count longhand (positive <integer>, initial 1), engine = "servo" with servo_pref = "layout.flexbox.balance" — not generated at all for Gecko.
• Gecko compatibility: gecko.mako.rs gains manual set/copy/reset/clone_flex_wrap mapping to structs::StyleFlexWrap (balance variants are unreachable on Gecko builds).

Differences from the DioxusLabs/stylo PR: the stylo_static_prefs/preferences.toml hunk (adding "layout.flexbox.balance" = false) has no equivalent here — that crate only exists in the standalone Stylo repo; in this tree static_prefs is Gecko's modules/libpref crate and the pref reference is servo-cfg'd out for Gecko. Everything else applied cleanly with only the style/ → servo/components/style/ path change.

Verification: ran properties/build.py for both gecko and servo products — both generate successfully; the gecko output contains no flex_line_count and uses the manual flex_wrap glue, the servo output contains the new longhand. Did not run ./mach build per instructions; the Servo cfg paths are not compiled in this tree.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/53c65c43aed94d578c1203fdc9994176
Requested by: @nicoburns

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