feat(atom): add dedupeWith combinator for custom write equivalence

[front-depiction]

Summary

Every atom already deduplicates writes using Equal.equals (Node.setValue in internal/registry.ts), but Equal.equals falls back to reference equality for values that don't implement Equal — plain objects, arrays, API payloads, derived collections. A structurally-identical new reference therefore re-notifies every subscriber and cascades invalidation downstream. This combinator sits in between no equality, and implementing custom Equal.Equal traits.

dedupeWith attaches a custom Equivalence<A> to an atom so structurally-equal writes become no-ops.

const user = Atom.make({ id: "u1", name: "Alice" }).pipe(
  Atom.dedupeWith((a, b) => a.id === b.id && a.name === b.name)
)

Design

• Atom<A> gains readonly eq: Equivalence<A>, defaulted via AtomProto to Equal.equals. No branching in the hot path — this.atom.eq(prev, next) is always a function call.
• dedupeWith is dual (data-first / data-last), returns Self → Self, and clones the prototype the same way setIdleTTL does (Object.assign(Object.create(proto), { ...self, eq })), preserving Writable / AtomRuntime proto methods.
• Node.setValue changes one line: Equal.equals(this._value, value) → this.atom.eq(this._value, value).
• Default behavior is preserved bit-for-bit: existing atoms with no dedupeWith call continue to dedupe via Equal.equals.

Test plan

New describe("dedupeWith") block in packages/atom/test/Atom.test.ts covers:

• plain-object atom without dedupeWith re-notifies on structurally-equal new references (status-quo witness)
• dedupeWith with a custom equivalence suppresses notify on structurally-equal writes and fires on genuinely new values
• dedupeWith does not suppress the initial value
• downstream derived atoms are not invalidated when the parent dedupes
• composes with setIdleTTL in either order (both eq and idleTTL preserved)
• default behavior preserved for primitives

Full suite: 104/104 pass. pnpm lint, pnpm circular, pnpm docgen clean. Changeset added (@effect-atom/atom: minor).