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

[nicoburns]

Summary

Implements CSS Flexbox Level 2 flex-wrap: balance and the flex-line-count longhand, Servo-only and gated behind a new layout.flexbox.balance pref (default false; always off for Gecko).

flex-wrap switches from a generated keyword enum to a custom bitflags type (matching the approach in DioxusLabs/firefox#1), used as both specified and computed value, parsing the Level 2 grammar nowrap | [ wrap | wrap-reverse ] || balance via the derived bitflags parser:

#[css(bitflags(
    single = "nowrap",
    mixed = "wrap,wrap-reverse,balance",
    validate_mixed = "Self::validate_and_simplify"
))]
#[repr(C)]
pub struct FlexWrap(u8);
bitflags! {
    impl FlexWrap: u8 {
        const NOWRAP = 0;
        const WRAP = 1 << 0;
        const WRAP_REVERSE = 1 << 1;
        const BALANCE = 1 << 2;
    }
}

validate_and_simplify rejects wrap wrap-reverse, rejects balance unless static_prefs::pref!("layout.flexbox.balance") is true (the pref lookup is servo-cfg'd, so it always fails on Gecko), and simplifies wrap balance → balance (canonical serialization: balance, wrap-reverse balance). nowrap balance is rejected by the bitflags grammar (nowrap is single).

Gecko's manual flex_wrap glue in gecko.mako.rs is removed; in the Firefox tree StyleFlexWrap becomes a cbindgen-exported type sharing this representation (see DioxusLabs/firefox#1).

flex-line-count is a new Servo-only longhand (positive <integer>, initial 1), servo_pref = "layout.flexbox.balance", not generated for Gecko.

Verified with cargo build --features servo and cargo check (no features), both clean.

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

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