Make float slot search height-aware (boxes must avoid floats over their full height)

[nicoburns]

Objective

Part 3/5 of the float placement fixes split from #1063 (stacked on #1065).

A box establishing a BFC must not overlap floats over its entire vertical extent, and a line box is shortened by any float its extent intersects — the slot search previously only considered the float insets of the starting segment.

• FloatContext::find_content_slot and find_bfc_slot (and the BlockContext wrappers) take a new height parameter and union float insets across all segments that [y, y + height) crosses. ContentSlot::height now reports the extent over which the slot's width remains valid.
• compute_block_layout estimates BFC-establishing children's height up-front from their style, then re-runs the slot search with the actual height after layout (re-laying the child out, up to 4 iterations, if the slot changes).
• The "below all floats" content slot now starts at the last segment's end rather than min_y, so content can't be placed higher than existing floats.

Fixes WPT css/CSS2/floats/floats-wrap-top-below-bfc-* (6 tests) and, with the companion Blitz inline changes (DioxusLabs/blitz#628), floats-wrap-top-below-inline-* (6 tests). Regression test added in tests/hand_written/floats.rs.

Context

Stack (bottom-up): rules 3&7 overflow (#1064) → margin collapse (#1065) → this PR → zero-height floats → float snapshot API. 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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