[nicoburns]

Summary

Adds dioxus-core (packages/core), a facade over the Core layer, so libraries can depend on the UI core without the Framework-level dioxus umbrella. The crate published as dioxus-core through 0.7 is now dioxus-vdom (#6021). The crate docs explain this.

dioxus-core

• Always depends on dioxus-vdom, dioxus-vdom-macro and dioxus-config-macros. It does not depend on crates it doesn't use: vdom-template, rsx, stores-macro and html-internal-macro are already reached through vdom, vdom-macro, stores and html.
• Features (the default is everything except ssr and wasm-split):

  macro      = []
  signals    = ["dep:dioxus-signals"]
  hooks      = ["dep:dioxus-hooks"]
  stores     = ["dep:dioxus-stores"]
  html       = ["dep:dioxus-html"]
  document   = ["dep:dioxus-document"]
  ssr        = ["dep:dioxus-ssr"]
  wasm-split = ["dep:wasm-splitter", "dioxus-config-macros/wasm-split"]

• Re-exports each crate as a module: core/dioxus_vdom, core_macro, signals, hooks, stores, html, document, ssr, config_macros, and wasm_split.
• prelude holds the Core-layer part of the old dioxus::prelude.

Macro roots: dioxus_root() in vdom-macro and router-macro (from #6023) now tries dioxus first, then dioxus-core, and handles renames of either. This means #[component(lazy)] and #[derive(Routable)] route splitting work for crates that depend only on dioxus-core. Routable still needs dioxus-router as a direct dependency.

dioxus

• Depends on dioxus-core (default-features = false) instead of vdom, vdom-macro, signals, hooks, stores, html, document, ssr, config-macros and wasm-splitter directly. Its features forward to the matching core features (wasm-split → dioxus-core/wasm-split).
• dioxus::prelude starts with pub use dioxus_core::prelude::*; and then adds the Framework items (launch, router, fullstack, manganis, logger and so on).
• Existing public paths are unchanged, including dioxus::core, dioxus::signals, dioxus::config_macros and dioxus::wasm_split.

Also: drops dioxus-html's unused manganis dev-dependency.

Tests:

• packages/core/tests/prelude.rs: covers a component, rsx! and a Store derive using only dioxus_core::prelude::*.
• packages/core/tests/wasm_split.rs: covers #[component(lazy)] through core.
• packages/dioxus/tests/core_facade.rs: checks that the umbrella paths still resolve.