feat: require a zero-error product typecheck and a test type-debt ratchet

Adds tsconfig.json (strict) over product code, fixes its remaining 90
diagnostics (type-only, interface corrections, and explicit narrowing),
and adds a typecheck job to the required free-tests aggregate running
bun run typecheck, the test-code ratchet (identity -> count baseline, fails
on new, repeated, or unlocked fixed diagnostics), and the lib/cso format
check. Reuses fixes from #2447 where they still applied.

browse/src/snapshot.ts

@@ -268,7 +268,7 @@ export async function handleSnapshot(
           const parts: string[] = [];
           let current: Element | null = el;
           while (current && current !== document.documentElement) {
-            const parent = current.parentElement;
+            const parent: Element | null = current.parentElement;
             if (!parent) break;
             const siblings = [...parent.children];
             const index = siblings.indexOf(current) + 1;