docs(setup): note ARM64 emulation expectations for demo stack

[haileyok]

Summary

• Add a short Apple Silicon subsection under Prerequisites in docs/DEVELOPMENT.md documenting that several demo-compose images (apache/druid, ghcr.io/ayubun/snowflake-id-worker, and the optional bigtable cloud-sdk emulator) are linux/amd64-only and run under Rosetta 2 on M-series Macs.
• Include the verbatim platform-mismatch warning text so users searching for the error land on this section.

No docker-compose.yaml changes — this is docs-only.

Test plan

• Spot-check that docs/DEVELOPMENT.md renders cleanly on the PR (markdown view).
• No SUMMARY.md update needed — the mdBook TOC only lists top-level pages, and the new subsection is in-page under the already-linked DEVELOPMENT.md.

Closes #96 (sub-item 5 of the friction log — platform mismatch warnings).

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