Add BlockContext::float_snapshot/restore_float_snapshot for speculative float placement

[nicoburns]

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. Used by DioxusLabs/blitz#628 (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 after rebasing onto v0.13-era main: blitz css/CSS2/floats+floats-clear = 247 PASS / 90 FAIL / 0 CRASH of 337 with the stack + DioxusLabs/blitz#628, vs 227 PASS / 110 FAIL on current blitz main (20 newly passing, 0 regressions).

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

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