Do not retain a flex basis measured during a previous layout

[mozzius]

Summary

Fixes #2019.

layout.computedFlexBasis persists across YGNodeCalculateLayout calls and is only cleared by markDirtyAndPropagate on that exact node. The resolved-flex-basis branch of computeFlexBasisForChild keeps whatever value is already stored, which is what lets a max-content measurement made earlier in the same layout survive into the definite-size pass (and is why flex: 1 inside an auto-height column means "content height"). The guard only checks whether a value exists, not which layout produced it.

When a later layout at a different root size hits an ancestor's measurement cache during the max-content pass, that subtree is skipped, the child's computedFlexBasis is never refreshed, and the definite pass retains a size measured against the previous available size. In React Native this shows up as a flex: 1 Text that keeps its landscape height after rotating back to portrait (react/react-native#58294).

Fix

Record whether a stored computedFlexBasis came from measuring the child's content (computedFlexBasisIsMeasured). In the resolved branch, a stored value that is measured and stamped with a previous generation is treated as stale: the child takes the measure path again, which does exactly what the skipped max-content pass would have done and normally hits the child's own measurement cache. A retained resolved basis is unchanged, so "definite from the start" containers still resolve flex: 1 to a basis of 0 as before, and nothing changes within a single layout.

Clearing the value instead (or enabling WebFlexBasis) is not a fix: the resolved branch then writes 0 with no measurement to follow and the child collapses to 0.

Test plan

• New test in tests/YGRelayoutTest.cpp: an 8-node tree (scroll container, flex: 1 column with a fixed-height sibling, two plain wrappers, a flex: 1 node with a text-like measure function) laid out at 400x800, 800x400, 400x800. Asserts the measured node returns to its first height. Fails on main, passes with this change.
• ./unit_tests Debug: all 843 tests pass. On main the new test fails (portraitHeight is 100, the relaid-out node is 60) and the other 842 pass.
• Standalone repro (https://github.com/mozzius/rotation-text-repro, yoga-harness/run.sh) built against this branch: two and three wrapper levels now return to their first height (they stay at the landscape height on main); one wrapper level passes on both.