Fix flex-shrink runaway when the sole shrinkable item has an aspectRatio basis

[sebholl]

Summary

Fixes #2038.

When the only shrinkable item on an overflowing row has its flex basis derived from aspectRatio, distributeFreeSpaceSecondPass divides the remaining free space by a 2^-19 residual left in totalFlexShrinkScaledFactors, and the item is measured at ~8.3e9 instead of 0. Sibling of #1665 / #1974 / #1783; the absolute 1e-6f guard from #1974 does not catch this residual.

Mechanism (full trace in #2038): calculateFlexLine accumulates round(-3 * 21.083334f) = -63.25. distributeFreeSpaceFirstPass then subtracts the same product inline, which clang contracts into a fused multiply-add using the unrounded product 63.2500019, leaving exactly 2^-19 = 1.9e-6. Building with -ffp-contract=off makes the bug disappear, which is also why it never shows in the wasm playground.

Two changes:

1. distributeFreeSpaceFirstPass: round the excluded product to float before subtracting it. This is the cause, and on its own it fixes the repro under the default -ffp-contract=on.
2. Replace the 1e-6f epsilon in distributeFreeSpaceSecondPass with an exact count of shrinkable items left on the line (FlexLineRunningLayout::numberOfShrinkableItems, kept in lockstep with the total). "Are any shrinkable items left?" is a counting question, and no absolute epsilon is right because the residual scales with flexShrink * flexBasis. This also keeps the fix correct under -ffp-contract=fast, where (1) alone can still be contracted across statements.

Left alone deliberately: the first-pass division (remainingFreeSpace / totalFlexShrinkScaledFactors inside distributeFreeSpaceFirstPass) is structurally exposed to the same issue if an item freezes mid-loop, but I have no repro for it, and a count-based guard cannot apply there because the current item is not yet frozen. Happy to add something if a maintainer prefers.

Test plan

• New tests/YGFlexShrinkAspectRatioBugTest.cpp (3 tests): the 4-node repro from flexShrink runaway (~8e9) when the sole shrinkable item has an aspectRatio basis; FMA residual slips past the 1e-6f guard from #1974 #2038, a sweep over shrink factors that currently diverge, and a basis guard. The first two fail on main and pass with this change.
• Full gtest suite: 859/859 pass (856 existing + 3 new), built with clang++ -std=c++20 -O2 (Apple clang, arm64).
• The flexShrink runaway (~8e9) when the sole shrinkable item has an aspectRatio basis; FMA residual slips past the 1e-6f guard from #1974 #2038 repro also clamps to 0 when built with -ffp-contract=fast.
• Existing YGFlexShrinkBorderBugTest (flexBasis+flexShrink+borderWidth result in an wrong value  #1665) still passes.

🤖 This PR includes AI-assisted changes via Claude Code.

[intergalacticspacehighway]

Thanks @sebholl for the analysis and repro! I've opened #2044, which sums the shrink factors of the unfrozen items instead of subtracting the frozen ones. That makes the total itself exact (0 when every item is frozen), so no separate count is needed, and it also fixes #931. I've also included your aspectRatio tests there.