fix(cloud): stream large session sharing imports

[Neonforge98]

Summary

• stream large cloud session uploads and downloads in bounded segments with resumable cursors and transient backoff
• import guest replays page-by-page into deferred SQLite state, then publish metadata atomically after finalization
• build turn indexes from the SQLite cursor instead of retaining a second full-history vector
• keep collaboration replays out of local cache-resave/in-flight reconciliation and open them from lightweight turn summaries
• bound Ctrl+5 payload retention and scope 5-second guest-share validation to the actually visible session tab

Cloud dependency

• cloud repo main: org2AI/ORGII-cloud-infra@e3616e464b345f493f120cefce138a832eb8bec5
• production migration 0002_bounded_session_event_pages.sql was applied successfully to the official Supabase project

Windows dual-instance Computer Use evidence

• primary and instance2 used isolated identities/databases; instance2 was a registered non-member in Personal Org
• imported the real 1.11 GB external Codex session twice from a guest share, including one Ctrl+5-instrumented clean run
• import peak: 1,357.9 MiB working set / 1,055.2 MiB private (old path: 2,776.0 / 2,518.2 MiB); settled after import near 850-920 / 519-577 MiB
• restart preserved the imported guest session; collapsed first open reached Latest round in 1.083 s at 727.8 / 414.0 MiB
• Ctrl+5 showed paired cloud_get_session_events_page and cache_append_session_event_import traffic and no es_save_to_cache
• active guest replay validation remained 12/min; after switching to Launchpad, clearing Ctrl+5, and waiting 16 seconds, no new API call appeared
• current lazy boundary is per turn: first open does not hydrate the giant body. Explicitly expanding the 94-minute turn reached about 1,583.5 / 1,237.6 MiB; within-turn scroll pagination is intentionally not claimed here

Verification

• pnpm exec tsc --noEmit
• focused Vitest: 79/79
• expanded Vitest: 108 files / 1,088 tests
• cargo test --manifest-path src-tauri/Cargo.toml -p session_persistence: 33/33
• cargo check --manifest-path src-tauri/Cargo.toml
• targeted ESLint and git diff --check
• pre-commit staged TypeScript check and Clippy (org2, session_persistence)
• parallel dual Tauri build completed for both identities