Bump Taffy to ac4c49c7 (Taffy #1290: cross-axis stretch in a definite flex container)

[nicoburns]

Summary

Re-pins taffy (Cargo.toml + Cargo.lock) to ac4c49c7, the tip of DioxusLabs/taffy#1290. No Blitz code changes.

That PR resolves a flex item's cross size of stretch against the container's cross size when sizing the flex lines, if that size is definite. Expected WPT changes:

• css-sizing/stretch/flex-line-003: 1/4 -> 4/4
• css-sizing/stretch/flex-line-004: 3/4 -> 2/4. Subtest 4 is fixed, subtests 1-2 now fail: the flex-flow: column wrap; width: min-content container is sized to the sum of its two lines (52px) where Chrome uses the widest item (26px), so the items now stretch to 52px. This is an existing difference in the intrinsic size of multi-line column containers.

The Taffy branch is based on Taffy main, which is ahead of Blitz's current pin, so the WPT results below also include DioxusLabs/taffy#1242 (all the css-sizing/stretch/block-height-* changes; see #1046).

The pinned commit is on the Taffy PR branch, not main. It needs re-pinning to the merged commit once that PR lands.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/3a06d93089f2434ba3519a7813dea1e8
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/3a06d93089f2434ba3519a7813dea1e8?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring