Docs: record RSC Rspack client refs investigation (#4269)

## Summary

- Adds an internal decision record for RORP RSC + Rspack
client-reference manifest generation.
- Captures the path from the initial `RSCRspackPlugin` work through the
`lazyCompilation` fix.
- Records why `addInclude`-only client inclusion is not yet a final
architecture and what remains open for Rspack/Shakapacker follow-up.

## Rationale

Fresh RSC + Rspack apps now work in normal `bin/dev`, but the
longer-term client-reference inclusion design is still unresolved. This
document preserves the investigation context so future work starts from
the known constraints instead of rediscovering them.

Related: #3488, #3553, #4200, #4227, #4243.

## Testing

- `git diff --check`
- `pnpm exec prettier --check
internal/analysis/rsc-rspack-client-reference-inclusion-decision-record-2026-06-27.md`
- pre-commit hook
- pre-push hook

## Changelog

- Not needed: internal analysis documentation only.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Added a new internal decision record outlining the history and current
status of the React Server Components + Rspack client-reference manifest
approach.
* Documents key tradeoffs, observed dev-server behavior, the current
mitigation in place, and open questions for longer-term improvements.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: ihabadham