[thedancingdeveloper]

Summary

Ports crates/fluent-core to the vendored upstream gpui snapshot on gpui-upstream (WI-1425, phase 1 of the fluent-crate port).

Stacked on PR #5 (gpui-upstream). Do not merge until #5 lands, and not before review.

No source change was required. WI-1350's drift report lists every gpui symbol fluent-core imports (Animation, AnimationExt, AnyElement, App, Background, ElementId, Global, Hsla, IntoElement, div, ease_out_quint, hsla, linear_color_stop, linear_gradient, prelude, px, rgb) as unchanged between the base fork and the vendored snapshot, and that holds: the crate compiles, its 22 unit tests pass, and clippy is clean against vendor/zed with no edits. This PR only records that in docs/gpui-api-drift.md.

• fluent-core was already a workspace member on gpui-upstream (WI-1347 did not drop it), so nothing was re-added.
• None of the re-applied patches (PRs SVG font-DB pre-warm (WI-1432) #6, Re-port element visual scale onto the vendored gpui #7, Video frames as standalone GPU textures #9) were needed; those branches were not merged in.
• No breaking changes to fluent-core's public API. WI-1426..1430 can port against it as-is.

Gates (Linux, local, rustc 1.99)

cargo check  -p fluent-core                         # ok
cargo test   -p fluent-core                         # 22 passed, 0 failed; 2 doc-tests ignored
cargo fmt    -p fluent-core --check                 # clean
cargo clippy -p fluent-core --all-targets --no-deps -- -D warnings   # clean

cargo clippy -p fluent-core --all-targets -- -D warnings (with deps) does not pass, but every error is inside the vendored closure (scheduler, ztracing, …), not in fluent-core. That is pre-existing on gpui-upstream and is WI-1472's to fix in the base CI; this PR does not touch it.

cargo clean was run after the gates.

Test plan

• Confirm CI on this PR (workspace clippy stays red until WI-1472; fluent-core's own code is clean)
• Reviewer confirms no behaviour change was intended and none was made