fix(spec): keep the blank-source refusal out of check:doc-authoring's sink closure

Interpolating the composed gate into the refusal put `featureGatePredicate` —
and through it the whole `PUBLIC_AUTH_FEATURES` registry — in a customer-facing
`message:` position, and that gate's per-module fixed point then swept the
registry's internal `notes` / `exempt.reason` prose as customer-facing text.
Measured: `pnpm check:doc-authoring` is green at the base commit and flagged
three pre-existing strings with the interpolation in.

The sentence now names the composed shape without interpolating the gate, which
is also how the two refusals beside it are written. No test pinned the wording.

Claude-Session: https://claude.ai/code/session_01JbZnqu8bt6YqfJsr9vaFb3
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

packages/spec/src/kernel/public-auth-features.ts

@@ -389,14 +389,23 @@ export function lowerRequiresFeature<T extends WithRequiresFeature>(
     ctx.addIssue({
       code: 'custom',
       path: ['requiresFeature'],
+      // ⚠ Deliberately does NOT interpolate `gate`. Doing so puts
+      // `featureGatePredicate` — and through it the whole
+      // `PUBLIC_AUTH_FEATURES` registry — in a customer-facing message
+      // position, and `check:doc-authoring`'s per-module fixed point then
+      // sweeps that registry's INTERNAL `notes` / `exempt.reason` prose as
+      // customer-facing text (measured: green at the base commit, three
+      // pre-existing strings flagged with the interpolation in). The concrete
+      // gate is one `featureGatePredicate` call away for anyone who wants it.
       message:
         '`requiresFeature` composes only with a CEL `visible` carrying a NON-BLANK `source`; this '
-        + '`source` is blank after trimming, so the composition would parenthesise nothing — '
-        + `\`( ) && ${gate}\` — which no CEL parse accepts on any scope. The gate would fault at `
-        + 'evaluation instead of gating: shown regardless of the flag where the surface is fail-soft, '
-        + 'hidden regardless of it where it is fail-closed. Either way the flag decides nothing, which '
-        + 'is the inert arrival ADR-0078 rejects. Drop the blank `visible` and `requiresFeature` emits '
-        + `\`${gate}\` alone, or put the predicate the gate should compose with in \`source\`.`,
+        + '`source` is blank after trimming, so composing the feature gate onto it would parenthesise '
+        + 'nothing — the predicate would read `( ) && ` followed by the gate — which no CEL parse '
+        + 'accepts on any scope. The gate would fault at evaluation instead of gating: the element is '
+        + 'shown regardless of the flag where the consuming surface is fail-soft and hidden regardless '
+        + 'of it where it is fail-closed, so the flag decides nothing — the inert arrival ADR-0078 '
+        + 'rejects. Drop the blank `visible` and `requiresFeature` emits the gate alone, or put the '
+        + 'predicate the gate should compose with in `source`.',
     });
     return rest as Omit<T, 'requiresFeature'>;
   }