Objective

Part 5/5 of the float placement fixes split from #1063 (stacked on #1067).

Adds a small public API for callers integrating inline layout (e.g. Blitz):

impl BlockContext<'_> {
    pub fn float_snapshot(&self) -> FloatContext;
    pub fn restore_float_snapshot(&mut self, snapshot: FloatContext);
}

An inline layout engine may need to place a float mid-line before knowing whether the line's content fits beside it (e.g. white-space: nowrap or unbreakable content discovered later on the line). The snapshot lets it speculatively place floats and roll them back if the line must be re-laid without them. DioxusLabs/blitz#628 currently pins #1067's head (so its revocable-float logic is temporarily removed); re-adding it on top of this API fixes WPT float-nowrap-3/9.html there.

Feedback wanted

This is a pragmatic clone/restore API; happy to reshape it (e.g. a transactional guard type) if preferred.

Context

Stack (bottom-up): rules 3&7 overflow (#1064) → margin collapse (#1065) → height-aware slots (#1066) → zero-height floats (#1067) → this PR. Together they supersede #1063 (closed). Latest WPT verification: blitz css/CSS2/floats+floats-clear = 246 PASS / 91 FAIL / 0 CRASH of 337 with #1066+#1067 + DioxusLabs/blitz#628 (19 newly passing vs blitz main's 227/110/0; +2 more — float-nowrap-3/9 — with this PR's API used).

Link to Devin session: https://app.devin.ai/sessions/2aa49a15ca484966b5b71922bfd2bb58
Requested by: @nicoburns