fix(tanstack-start): strip RegExp in toSerializable to unblock SSR bootstrap

Run #4's Map→object fix for versionViewData.clientSchemaMap landed but
the versions diff-view shard stayed 0/40. The actual blocker was a
separate non-serializable value in the payload — RegExp literals from
rich-text feature markdownTransformers (e.g. UPLOAD_PLACEHOLDER_REGEX,
/!\[([^\]:]+):([^\]]+)\]\(\)/).

toSerializable previously preserved RegExp unchanged. seroval then
serializes the regex into the SSR bootstrap script, but with doubled
backslashes in source (emits `/!\\[.../` instead of `/!\[.../`). That
is syntactically invalid JS regex ("Unmatched ')'"), so the script
that populates `window.$_TSR` throws mid-stream. TanStack hydration
then fails with "Invariant: Expected to find bootstrap data on
window.$_TSR", the client re-renders the tree empty, and the diff
wrapper never reaches the DOM — hence every test in
`versions/e2e.spec.ts › Versions diff view` timed out waiting for
`.render-field-diffs`.

Next.js doesn't hit this because its server→client transport is JSON,
which silently drops RegExp. Strip RegExp here to match that behavior.
Rich-text transformer regexes are server-only (markdown import/export)
and aren't referenced by buildVersionFields or any client renderer,
so the loss is harmless.

Verified locally against test/versions/config.ts: before, the page
stuck at the Invariant error with an empty client tree; after, the
bootstrap parses, hydration completes, and .render-field-diffs
renders through SSR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: r1tsuu

packages/tanstack-start/src/utilities/toSerializable.ts

@@ -50,10 +50,19 @@ function stripUnserializable(
     return cache.get(obj)
   }
 
-  if (obj instanceof Date || obj instanceof RegExp) {
+  if (obj instanceof Date) {
     return obj
   }
 
+  // RegExp: seroval's SSR bootstrap emits regex literals with broken escape
+  // sequences (e.g. `/!\\[.../` instead of `/!\[.../`), crashing the client
+  // script that populates `window.$_TSR` and preventing hydration. Rich-text
+  // feature transformer regexes aren't needed on the client (they drive
+  // server-side markdown import/export), so strip them here.
+  if (obj instanceof RegExp) {
+    return undefined
+  }
+
   ancestors.add(obj)
 
   if (obj instanceof Map) {