gpui: Add ReadEntity and Projection entity handles

[mikayla-maki]

Completes the entity handle lattice with read-only and lensed capability types:

	whole entity	lens
read + write	Entity<T>	ProjectionMut<P>
read only	ReadEntity<T>	Projection<P>

each with a weak twin (WeakReadEntity, WeakProjection, WeakProjectionMut), and conversions flowing write→read and whole→lens.

• ReadEntity<T> restores the read/write distinction that shared-mutability entities erase: holders can read (and be re-rendered via access tracking) but cannot update or notify, making an entity's writer set auditable.
• Projection<P>/ProjectionMut<P> are source-erased lens handles over part of an entity's state, so components can accept any P-shaped state (impl Into<ProjectionMut<String>>) without dictating how callers store it. P may be unsized (Projection<str>). Writes route through the source entity and always notify it.
• Lens projections are constructed during render, via Window::use_projection/use_projection_mut (and use_keyed_* variants for loops), or by converting an Entity<P> with From. Each hook call site gets a memoized relay entity (like use_state) as the projection's identity, so sibling projections of different fields of one entity don't collide when used as view identities; the relay forwards source notifications so observation and caching keyed on the projection work. There is deliberately no way to construct a lens projection outside a render context — state that needs identity independent of any view should be a proper entity.
• The project! macro derives both lenses of a mutable projection from a single field path: project!(window, cx, &person, address.city).
• Lenses are non-capturing function pointers (capturing closures are rejected at compile time), stored type-erased next to a monomorphized trampoline, making projections allocation-free at construction beyond the relay memoized per call site.
• Supporting APIs: AnyEntity::downcast_ref (backed by #[repr(transparent)] on Entity<T>) and EntityMap::read_any, so projection reads and writes avoid handle clones and refcount traffic.

Deliberate omissions, discussed and deferred: no equality-based notify dedupe (projecting from a notify-happy entity is a state-modeling smell — split the hot field into its own entity), and no observe/subscribe on read handles yet.

Release Notes:

• N/A

[SomeoneToIgnore]

To keep the PR queue cleaner, will close this PR for now, as this is a draft still with no changes for over 3 weeks.