refactor: remove global mutable state — registry guard, compat warning, stats

predicates.ts: Replace defaultPredicatesRegistered boolean guard with
  sentinel predicate check inside the registry. registerDefaultPredicates()
  is now always safe to call — no module-scope flag that can drift out
  of sync with the actual registry state. clearPredicateRegistry() no
  longer needs to manually reset a flag.

extraction.ts:
  - Replace compatibilityWarningEmitted process-singleton boolean with
    WeakSet<ImhotepUi> (warnedUis). Each ImhotepUi instance now gets its
    own compatibility warning, fixing the bug where two pages would share
    a single warning gate.
  - Export resetExtractionPathStats() for test isolation of fast-path
    and CDP fallback counters.

598 SDK + 3 conformance + 57 E2E = 658 tests pass.

packages/imhotep-playwright/src/extraction.ts

@@ -83,6 +83,12 @@ const extractionPathStats = {
   cdpFallbacks: 0,
 }
 
+/** Reset extraction path counters (for test isolation). */
+export function resetExtractionPathStats(): void {
+  extractionPathStats.fastPathHits = 0
+  extractionPathStats.cdpFallbacks = 0
+}
+
 const pageCacheNamespace = new WeakMap<Page, string>()
 let pageCacheNamespaceCounter = 0
 
@@ -95,7 +101,7 @@ function getPageCacheNamespace(page: Page): string {
 }
 
 let extractionStatsHookInstalled = false
-let compatibilityWarningEmitted = false
+const warnedUis = new WeakSet<ImhotepUi>()
 
 function maybeInstallExtractionStatsHook(): void {
   if (extractionStatsHookInstalled) return
@@ -1980,10 +1986,10 @@ export function buildCompatibilityReport(ui: ImhotepUi): CompatibilityReport {
 }
 
 export function maybeEmitCompatibilityWarning(ui: ImhotepUi): void {
-  if (compatibilityWarningEmitted) return
+  if (warnedUis.has(ui)) return
   const report = buildCompatibilityReport(ui)
   if (report.ok) return
-  compatibilityWarningEmitted = true
+  warnedUis.add(ui)
   const failed = report.checks.filter((c) => !c.ok)
   const detail = failed.map((c) => `${c.id}: ${c.details}`).join(' | ')
   const fingerprints = report.packageFingerprints