fix(sessions): stabilize durable history reconciliation

[bobleer]

Summary

• Port 1f29e82a1 and e666b6e62 from 1.0.0-explore onto the latest main.
• Make Runtime completion authoritative for settled Turn content and emit a per-Turn durable history fence after persistence succeeds.
• Repair Desktop, Peer Device, and Mobile projections from persisted history without dropping a newer live Turn.
• Cache read-only snapshot managers and load large snapshot metadata indexes on bounded blocking workers to reduce restore stalls.

Type and Areas

Type: regression fix / performance fix

Areas: Rust core, event contracts, desktop/Tauri, Remote Connect, Web UI, Mobile Web, Peer Device Mode, architecture docs

Motivation / Impact

Terminal stream chunks can be missed while a window is suspended, a remote controller reconnects, or persistence races the frontend completion finalizer. A client could therefore keep a completed assistant response prefix, or a late history fence could erase the projection of a newer running Turn. Large snapshot indexes also made the repair path hold session coordination for seconds.

This change makes the persisted terminal Turn the repair authority, adds a backward-compatible settled-Turn fence, preserves newer live projections, and avoids repeatedly loading snapshot metadata.

Verification

• cargo check -p bitfun-events — passed
• cargo check -p bitfun-desktop — passed
• cargo test -p bitfun-core --no-default-features --features agent-runtime --lib stale_projected_turn_saves_cannot_overwrite_runtime_recovery_state — passed
• cargo test -p bitfun-core --no-default-features --features agent-runtime --lib completion_replaces_a_projected_text_prefix_with_runtime_generation_content — passed
• cargo test -p bitfun-core --no-default-features --features agent-runtime --lib completed_persisted_turn_emits_a_durable_history_fence — passed
• cargo test -p bitfun-core --no-default-features --features agent-runtime --lib view_manager_is_cached_and_superseded_by_a_later_writer — passed
• cargo test -p bitfun-services-integrations --no-default-features --features remote-connect --lib remote_connect::tests:: — 9 passed
• cargo test -p bitfun-services-integrations --no-default-features --features remote-connect --test remote_connect_contracts — 55 passed
• pnpm --dir src/web-ui run gen:types && pnpm run type-check:web — passed
• pnpm --dir src/web-ui run test:run src/flow_chat/services/flow-chat-manager/EventHandlerModule.test.ts src/flow_chat/store/FlowChatStore.test.ts — 172 passed
• pnpm --dir src/mobile-web run type-check && pnpm run build:mobile-web — passed
• git diff --check upstream/main...HEAD — passed

Remote scenario coverage:

• Local/Desktop: Core unit coverage and Desktop compile check.
• Remote control: Remote Connect unit/contract coverage and Mobile Web type/build checks.
• Peer Device Mode: settled local/Peer projection regression coverage in FlowChatStore tests.
• Remote workspace and Detached Dispatch: not manually exercised; the event field and poll replacement field are optional for cross-version compatibility.

No manual browser/device run was performed.

Reviewer Notes

• settled_turn_id is optional and omitted when absent, so older event payloads continue to deserialize.
• message_snapshot is optional; the additive delta remains populated for older Mobile Web clients.
• The second commit is a follow-up to the first and should be reviewed/applied as a pair.
• AI-assisted port; verification level is focused automated testing as listed above.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.