Overload cached for non-class use

[NullVoxPopuli-ai-agent]

Implements the standalone form of cached proposed in Overload cached for non-class use — the caching companion to RFC 1071's overloaded tracked, and mirrors that implementation (#21471) throughout.

import { tracked, cached } from '@glimmer/tracking';

const count = tracked(0);
const doubled = cached(() => count.value * 2);

doubled.value; // read the cached value, entangling with any tracking context
doubled.get(); // function shorthand for reading

• New cachedValue(fn, options) in @glimmer/validator, returning RFC 1071's ReadOnlyReactive — no set/update/freeze, it has no storage of its own. Backed by createCache/getValue. Per RFC review there is no new public type: get() joins the ReadOnlyReactive interface (an RFC 1071 oversight) and the implementation class stays internal.
• options.description labels the cache for debugging. (An earlier revision proposed options.equals; removed per RFC review — calling fn is the expensive part, so re-running it only to discard the result has unclear benefit.)
• @ember/-internals/metal cached dispatches between the two forms: a legacy decorator invocation always receives three arguments, so cached(fn) / cached(fn, options) can never collide with @cached on a getter (including static getters). Decorator behavior is unchanged.
• The docs blocks gain a "Standalone usage" section; the existing Reactive/ReadOnlyReactive/TrackedValue type re-exports from @glimmer/tracking are unchanged.
• Tests at every layer, mirroring Implement RFC #1071: overloaded tracked #21471: @glimmer/validator unit tests, glimmer integration-tests rendering tests, metal standalone + assertion tests, an ember rendering test (Component Standalone Cached Values), and type-tests covering overload resolution, read-only value, and the options object.

Draft until the RFC is proposed/accepted upstream.

Test results: filter=cached 33/33 pass, filter=tracked 273 pass / 3 skip, type-check clean, lint:docs pass, prettier/eslint clean.

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