✨ Extend tsconfig from @tsconfig/node26 and @tsconfig/strictest

- Replace hand-rolled strict flags with @tsconfig/strictest
- Pick up Node 26 module/lib/target from @tsconfig/node26
- Add exactOptionalPropertyTypes (the main strictness gain)
- Pin target to es2024 to remain conservative for the published build

package-lock.json

@@ -9,6 +9,8 @@
             "version": "0.0.0",
             "license": "MIT",
             "devDependencies": {
+                "@tsconfig/node26": "^26.0.1",
+                "@tsconfig/strictest": "^2.0.8",
                 "@types/node": "^26.4.1",
                 "c8": "^12.0.0",
                 "check-outdated": "^3.0.0",
@@ -1398,6 +1400,20 @@
                 "node": "^20.19.0 || >=22.12.0"
             }
         },
+        "node_modules/@tsconfig/node26": {
+            "version": "26.0.1",
+            "resolved": "https://registry.npmjs.org/@tsconfig/node26/-/node26-26.0.1.tgz",
+            "integrity": "sha512-NQT8JUFDKeAIWpR1dakeF534msQAsoSLr1WpKdRDt3sCvk2fsFK3EVKUBG5xKYVamiRO6yTfniiyvCaqOGgn/g==",
+            "dev": true,
+            "license": "MIT"
+        },
+        "node_modules/@tsconfig/strictest": {
+            "version": "2.0.8",
+            "resolved": "https://registry.npmjs.org/@tsconfig/strictest/-/strictest-2.0.8.tgz",
+            "integrity": "sha512-XnQ7vNz5HRN0r88GYf1J9JJjqtZPiHt2woGJOo2dYqyHGGcd6OLGqSlBB6p1j9mpzja6Oe5BoPqWmeDx6X9rLw==",
+            "dev": true,
+            "license": "MIT"
+        },
         "node_modules/@types/istanbul-lib-coverage": {
             "version": "2.0.6",
             "resolved": "https://registry.npmjs.org/@types/istanbul-lib-coverage/-/istanbul-lib-coverage-2.0.6.tgz",