docs(website): document Astro 7 migration plan

[100gle]

Summary

• capture the verified Astro 7 and Starlight migration baseline
• document compatibility findings, rollout layers, risks, and rollback gates
• keep the implementation reviewable as a five-layer stack

Stack

1. This PR — migration plan
2. chore(website): upgrade the Astro toolchain #16 — Astro 7 toolchain
3. perf(website): prerender and modernize site runtime #17 — disable unused Astro sessions
4. feat(website): upgrade the Blog with Starlight #18 — unify Blog with Starlight
5. ci(website): verify static Cloudflare deployment #19 — Cloudflare deployment validation

Verification

• official Astro/Starlight/Cloudflare migration guidance cross-checked
• repository baseline and route inventory captured before implementation

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Project	Deployment	Actions	Updated (UTC)
bub	Ready	Preview	Aug 12, 2026 2:00pm

[100gle]

Migration research should not be committed. Closing this layer and rebasing the implementation stack directly onto main.