Retain public accessibility for witnesses of external protocol requirements

[danwood]

Symptom

A public member declared in a protocol extension is reported as redundantly public — even when it is the witness for a requirement of an external public protocol such as the standard library's Identifiable. Removing the public modifier, as the report suggests, breaks compilation:

public protocol StatusRepresentable: RawRepresentable, CaseIterable, Identifiable, Hashable, Sendable
    where RawValue == String {}

// `id` is the witness for Identifiable.id (a public protocol requirement).
public extension StatusRepresentable {
    var id: String { rawValue }   // flagged "redundant public"
}

Downgrading the extension from public yields: "property 'id' must be declared public because it matches a requirement in public protocol 'Identifiable'".

Root cause

RedundantExplicitPublicAccessibilityMarker.validateExtension(_:) marks a public extension as redundantly public whenever the extended type is itself flagged redundant. It does not consider that a member of the extension may be the witness for a requirement of an external public protocol, which obliges the witness to remain at least as accessible as the requirement.

The witness relationship survives in the graph as a related reference on the member whose USR does not resolve to any local declaration (because the protocol — e.g. Identifiable — is external). The existing cross-module exemption logic only covers witnesses of local protocols, so external-protocol witnesses fall through.

Fix

Before marking an extension redundantly public, check whether any of its members is the witness for an external protocol requirement: a related reference of a protocol-member-conforming kind, with a matching name, whose USR resolves to no local declaration. If so, the extension is exempt. This is general — it covers any witness for any external public protocol requirement supplied in a protocol-extension default implementation, with no special-casing of Identifiable or id.

Adds an accessibility test covering a public protocol refining Identifiable whose id witness is supplied in a default-implementation extension.