Fix floats × inline content / line box WPT failures

[nicoburns]

Summary

Fixes WPT failures in the "floats × inline content / line boxes" group, on top of #625. css/CSS2/floats + css/CSS2/floats-clear: 195 → 206 passing of 325 run (0 crashes), no regressions. (Locally-measured baseline for #625 was 195/130; the reported 192/133 could not be reproduced.)

Newly passing:

css/CSS2/floats/float-in-inline-001.html
css/CSS2/floats/float-in-inline-002.html
css/CSS2/floats/float-nowrap-4.html
css/CSS2/floats/floats-placement-vertical-001b.xht
css/CSS2/floats/floats-placement-vertical-001c.xht
css/CSS2/floats/floats-wrap-bfc-with-margin-007.tentative.html
css/CSS2/floats/zero-width-floats.html
css/CSS2/floats-clear/floats-149.xht
css/CSS2/floats-clear/margin-collapse-033.xht
css/CSS2/floats-clear/margin-collapse-034.xht
css/CSS2/floats-clear/margin-collapse-035.xht

(floats-006.xht, float-nowrap-3.html, float-nowrap-9.html were fixed as regressions of intermediate states and stay passing.)

Blitz changes

1. Floats inside inline content stay in the IFC (float-in-inline-001/002): collect_complex_layout_children treated a floated child as block-level (its blockified display interrupted the inline formatting context, splitting it into anonymous blocks). Floated children are now classified as inline-level and become InlineBoxKind::CustomOutOfFlow inline boxes handled by the float machinery; Node::is_or_contains_block likewise no longer counts floated (or absolutely positioned) descendants as in-flow blocks.

2. Text index / whitespace handling around out-of-flow inline boxes (floats-149, floats-006): Parley doesn't commit pending text when an out-of-flow inline box is pushed, so the box's text index ignored preceding uncommitted text. push_inline_box_with_index_fixup commits pending text via an empty style-modification span — but only when it contains non-whitespace (whitespace-only text must stay uncommitted so it collapses with surrounding text; committing it created a phantom root-styled run whose line-height misplaced later floats in floats-006). Committing sets Parley's "span first" flag which swallows a following collapsible space, so that space is re-inserted explicitly when needed.

3. Speculative retry when Parley rewinds past a placed float (float-nowrap-3/4/9): with white-space: nowrap, Parley can rewind content that precedes an already-placed float onto a later line, re-encountering the float. Line breaking now runs in a retry loop (max 4 attempts):

   • a re-encountered float is not placed twice; if its line moved down, a retry is requested with the higher min-y;
   • after each pass, floats placed above a line box containing content that precedes them (CSS2 §9.5.1 rule 6) also force a retry;
   • each retry restores taffy float state via the new BlockContext::snapshot_float_state/restore_float_state API, so discarded attempts leave no phantom floats.

Taffy pin

Bumps the taffy pin to DioxusLabs/taffy#1070 (zero-width float vertical participation, real-float-edge tracking for BFC slots, and the float state snapshot/restore API). Once that merges, the pin can move back to main.

Remaining failures in this group

block-in-inline-become-float.html, float-nowrap-hyphen-rewind-1.html, float-no-content-beside-001.html, floats-line-wrap-shifted-001.html, intrinsic-size-float-and-line.html, zero-space-between-floats-003.html, floats-029/030/031/124/132.xht (line boxes shortening beside floats mid-paragraph — needs per-line width recomputation that accounts for line height before the line is broken). float-nowrap-5/6, floats-036, floats-114 are assigned to a separate session.

cargo fmt and cargo clippy --workspace are clean.

Link to Devin session: https://app.devin.ai/sessions/ca393611e919410aaa93e816fdf90a54
Requested by: @nicoburns

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