Implement the layout-affecting parts of CSS layout and paint containment

[nicoburns]

Objective

Add a contain style to Taffy with layout and paint flags, and implement their layout effects. Based on the previously-reverted #1106, but with all size / inline-size containment stripped out.

New style API:

pub struct Contain(u8); // bitflags: NONE, LAYOUT, PAINT, CONTENT (= LAYOUT | PAINT)

pub struct Style { pub contain: Contain, ... }

pub trait CoreStyle {
    fn contain(&self) -> Contain { Contain::NONE } // defaulted
}

Implemented effects (both layout and paint, since both force an independent formatting context):

• Block: a contained box establishes a new BFC — margins don't collapse with its children, it contains its own floats, and it avoids external floats
• Overflow: a contained box's overflowing content no longer contributes to its ancestors' scrollable overflow regions (layout containment treats it as ink overflow; paint containment clips it). The box's own scrollable_overflow_rect still includes it
• layout only: the box's baseline is suppressed for baseline-alignment purposes in block, flexbox and grid containers (a synthesized border-box baseline is used where one is required)

CSS parsing (parse feature) accepts none | content | [ layout || style || paint ]; style is accepted and ignored (no layout effect); strict / size / inline-size are rejected as size containment is not implemented.

Also fixes a preexisting block/float bug this work surfaced: overflowing in-flow content of a nested block no longer contributes to the height of its BFC root as if it were floated content (float_content_contribution now starts at NEG_INFINITY instead of the content edge).

CHANGELOG.md is updated under Added/Fixed.

Context

• Inspiration commit (reverted): 20f04b9 / Implement the layout-affecting parts of the CSS contain property #1106
• Gentest support added (contain attribute in fixtures/helper/xml runner) with new generated fixtures under test_fixtures/contain/ covering margin collapsing, float containment/avoidance, contain: content, and flex baseline suppression for both layout and paint
• A companion Blitz PR wires up contain: layout / contain: paint from Stylo (link to follow)

Feedback wanted

• Whether Contain as a minimal hand-rolled bitflags type (rather than a bitflags! dependency or an enum) is the preferred representation
• The scrollable-overflow semantics: both containment types stop propagation to ancestors but keep the box's own scrollable_overflow_rect unclipped — for paint a consumer may additionally want to clip when painting

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/c903534a83f54a6daf2f68f4cc38cf72
Requested by: @nicoburns

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