Font-wait counts only actively-loading faces; excuse aborted fetches whose file exists

- Lazy-loading semantics: an @font-face never matched by rendered text
  stays "unloaded" forever - the readiness poll now waits only on faces
  in "loading" state, ending the instant fetches settle (removes ~3s of
  dead deadline per viewport on affected pages); unreferenced faces log
  per-face informationally instead of warning
- Asset gate counts a failure only for real HTTP >= 400 or a failed
  fetch whose target file is genuinely absent from the export - aborted
  srcset-candidate image fetches with valid files on disk are excused,
  matching the existing streaming-media treatment

458 tests pass (11 new), typecheck clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

compiler/src/validate/gates.ts

@@ -222,7 +222,7 @@ export function gate2Assets(assetGraph: AssetGraph, fontGraph: FontGraph, gen: {
   if (zeroByte > 0) issues.push(`${zeroByte} zero-byte downloaded assets`);
   if (skippedNoReason > 0) issues.push(`${skippedNoReason} skipped assets without reason`);
   if (gen.remoteRefs.length > 0) issues.push(`${gen.remoteRefs.length} generated refs point to remote origin`);
-  if (gen.failed404.length > 0) issues.push(`${gen.failed404.length} generated asset refs 404`);
+  if (gen.failed404.length > 0) issues.push(`${gen.failed404.length} generated asset refs missing (HTTP >= 400 or file absent from export)`);
   const fontsResolvedOrFallback = fontGraph.entries.every((f) => f.status === "resolved" || (f.status === "fallback" && f.reason));
   if (!fontsResolvedOrFallback) issues.push("font declarations not resolved/fallback-recorded");
   return {