Remove generated error codes (#97687)
### Why?
Should come up with better solution that does not block PRs with git
conflict
x-ref:
https://vercel.slack.com/archives/C02CDC2ALJH/p1785263902728189?thread_ts=1785263687.502649&cid=C02CDC2ALJH
### How?
- Delete `errors.json`, the error-code SWC plugin, generated WASM, merge
driver, and validation/build tooling.
- Stop attaching error codes to server-rendering digests, redboxes, and
telemetry; native `Error.code` and `Error.name` remain available where
applicable.
- Remove the development-overlay error feedback UI, middleware, and
telemetry event that depended on stable codes.
- Update fixtures, snapshots, and guidance for code-free errors and
numeric-only digests.
<!-- NEXT_JS_LLM -->
