iris: factor the tabs example's widget tree into tabs-ui (I2, part 2)

The pass condition for android-view backend (RUST.md's I2) is that the
tabs example itself, text field included, runs there -- not a second
demo with the same shape. tabs-ui/src/lib.rs is that widget tree moved
out of examples/tabs/main.rs into a small crate generic over `Rsc:
HasEvents` and `Rsc::State: FocusHost`, so the winit example and the
upcoming android-app cdylib both call the same `build()` rather than
carrying two copies. Nothing in it names either backend.

Verified: the winit tabs example still renders pixel-identically via
run-headless.sh (27266 bytes, unchanged), tabs-ui cross-compiles clean
for x86_64-linux-android alongside iris, and host build/clippy/fmt/
tests are unaffected.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

iris/Cargo.toml

@@ -42,9 +42,13 @@ send_wrapper = "0.6.0"
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread", "time"] }
+# The tabs example's widget tree. A dev-dependency cycle back to this
+# package is fine -- cargo excludes dev-dependencies from the graph used
+# to build the library itself, so this only matters for `--examples`.
+tabs-ui = { path = "tabs-ui" }
 
 [workspace]
-members = ["core", "macro"]
+members = ["core", "macro", "tabs-ui"]
 
 [workspace.package]
 version = "0.1.0"