Add refract: a store/lens-first reactive UI runtime experiment

[ealmloff]

Summary

New workspace crate refract (publish = false): a from-scratch Dioxus-like reactive runtime built around stores and lenses, inspired by ealmloff/qk. See refract/DESIGN.md for the full rationale — especially the memo/resource design.

Core model:

• Store<T> roots live in a thread-local generational slab; each value is a Box<RefCell<dyn Any>>, so guards are zero-copy Ref/RefMut projections (Ref::map) — no clones, no Rc<T> per field.
• Lens/IndexLens compose over any Readable, carrying a structural path (Vec<u32>). Invalidation uses path-prefix overlap: writing store.field wakes readers of field and of the whole store, but not sibling fields; todos[3] doesn't wake todos[4].
• Tracking via guards, qk RwTrack-style: ReadGuard::deref subscribes; WriteGuard::deref_mut flags the write, and notification happens on guard drop after releasing the RefMut. A WriteGuard that is only Deref'd never notifies.

Memo (Clean/Check/Dirty with version counters):

• Lazy: writes push Dirty to direct subscribers and Check transitively; ensure() walks deps and only recomputes when a dep's version actually changed.
• Equality-gated: version bumps only if the new value != old, so diamond graphs cut off.
• Memos are brought up to date before handing out a ReadGuard, so recomputation never needs &mut while a lens guard into the memo is live. Cycles panic deterministically (computing flag), and holding a guard across a write to the same cell is a deterministic RefCell panic, never UB.

Resource (the Pin-sensitive part):

• Value cell (ResourceState<T>: Pending/Ready/Reloading) and the driver Pin<Box<dyn Future<Output = ()>>> are stored separately — polling needs &mut on the future while lenses may hold Refs on the state, and the future may be self-referential after first poll so it is never moved out of its box.
• Dep changes drop the old future (cancellation), preserve stale data as Reloading(T), and spawn a new one. Only reads in the synchronous source closure are tracked.

Also: eager Effects with ownership scopes (children/deps torn down on rerun, queue processed shallowest-first so parents dispose stale children), a minimal retained DOM (el/text/dyn_text/dyn_attr/dyn_children bound via effects, no VDOM), 15 integration tests, and counter/todos/weather examples.

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features (15 refract integration tests) and cargo test -p refract --doc
• RUSTDOCFLAGS=-Dwarnings cargo doc --no-deps --all-features -p refract
• cargo run -p refract --example counter|todos|weather

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/edc42af2e89248a280abfa6508da3a43
Requested by: @ealmloff

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