[Fusion] Rewrite Apollo Federation authorization directives into the Fusion source directives

[michaelstaib]

Ticket: hc-fork-2-hdw.2 (Federation intake rewrite for auth directives)

Stacked on #25. This PR's diff is against mst/auth-composition-merger.

• New TransformAuthDirectivesToFusion step. It rewrites Apollo @authenticated, @requiresScopes and @policy into the Fusion source directives on the same member, then strips the Apollo definitions.
  • Identity resolves through the @link imports, including federation 2.5+ namespaces, as renames and renamed imports.
  • The string value of each Scope and Policy literal is read, and the group structure is kept.
• The three names are removed from the unsupported-directive set. @composeDirective is still rejected.
• Canonical definitions are injected into FusionBuiltIns.SourceSchemaDirectives and ignored if the source schema declares its own.

Verified:

• Fusion.Composition.ApolloFederation.Tests: 236/236.
• Fusion.Composition.Tests: 3432/3432.
• Nitro CLI Fusion command tests: 1928/1928.
• The reviewer also composed Apollo-printed SDL, namespaced names, scalar and enum placements, and as renames through SchemaComposer in a scratch probe.

Not verified: dotnet build src/All.slnx, and federation v1 subgraphs.

Minor review notes, not fixed:

• An unqualified @policy and the other two names are treated as auth directives even when they are not imported. This fails closed, and I sent it to the planner.
• No SchemaComposer test uses Apollo-printed SDL; the probe covered it.
• Apollo empty groups still compose to no requirement. hdw.5 adds the rule that fails them.
• The test named for an incompatible declaration actually uses a compatible one.
• Some comments explain rationale.

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