0.8: migration guide + docs rewrite

[joaomdsg]

Full-breakage release needs a complete map.

Scope: MIGRATION.md v1->v0.8 (every removed/renamed symbol -> replacement); README rewrite for the v2 core; refresh or archive DESIGN.md/COMPARISON.md/CONVERGENCE.md/FORUM-DESIGN.md; state deferrals explicitly (keyed live islands, backplane, showcase -> 0.9+).

Acceptance: every exported v1 symbol accounted for in MIGRATION.md; no doc claims contradicting the shipped code (doc-drift check).