fix(scaffold): repair new-demo registry insertion (anchor drift, CRLF, missing updatedAt)

[linqi88]

Problem

npm run new-demo — step 1 of the documented "Add your own showcase" flow — fails on current main (verified at 0d75d2b):

1. The entry-insertion anchor looks for export const demos: DemoEntry[] = [ + newline, but the registry now declares const authored: DemoEntry[] = [ (the exported const is [...authored], whose line never ends with an open bracket). The anchor can never match → "insert the entry manually".
2. Both insertion anchors are \n-exact, so on CRLF working trees (Windows default) even the import insertion silently fails — producing a registry that references an unimported factory and fails tsc.
3. A failed run leaves src/demos/<id>/ behind, so the retry hits "already exists — pick a different id".
4. The scaffolded entry is missing the required updatedAt field, so npm run build fails even after the anchors are fixed.

Fix

• Accept either const authored / export const demos as the entry anchor; tolerate \r?\n in both anchors.
• Fail loudly when the import anchor doesn't match instead of writing a broken registry.
• Stamp updatedAt on the scaffolded entry.

Verification

• npm run new-demo -- pr-scaffold-check "Scaffold Check" object completes and inserts the entry into const authored; npx tsc --noEmit is clean; probe artifacts removed afterwards.
• Repeated with a CRLF-converted registry.ts to confirm the tolerance.