Use explicit ts specifiers for declarations

Restore explicit .ts relative specifiers in source and enable rewriteRelativeImportExtensions so emitted JS uses .js while declarations keep explicit .ts specifiers. Add a NodeNext declaration-consumer gate to prevent extensionless declaration regressions.

tsconfig.base.json

@@ -10,6 +10,8 @@
     "incremental": true,
     "skipLibCheck": true,
     "esModuleInterop": true,
+    "allowImportingTsExtensions": true,
+    "rewriteRelativeImportExtensions": true,
     "verbatimModuleSyntax": false,
     "strict": true,
     "noUncheckedIndexedAccess": true,