perf(remote): load session history pages on demand

[wgqqqqq]

Summary

Opening a long remote conversation currently reads and projects every persisted turn before cutting the first stream page. Load historical turns on demand until the requested page reaches its event/byte budget instead. Android, iOS and HarmonyOS keep the existing read_stream protocol.

Type and Areas

Type: Performance / bug fix.

Areas: Rust session persistence and Remote Connect host streams; native mobile transport regression tests.

Motivation / Impact

An opt-in benchmark compares both production persistence/projection/stream paths against the same 128-turn fixture (128 KiB input per turn). Fresh hubs are used for each of five samples, and the first-page payloads are asserted identical apart from revisions.

Local first-page work	Full materialization	On-demand history
Median, debug build	1,580 ms	38.5 ms
Turn bodies read	128	3

This is approximately 41x faster in this local synthetic fixture, not an end-to-end phone/relay measurement. All five samples are printed by the reproducible benchmark; there is no machine-dependent timing assertion in CI.

Verification

• cargo test --locked -p openbitfun-core --no-default-features --features remote-connect,git --lib agentic::persistence::manager::tests:: — 57 passed, 2 opt-in tests ignored.
• Same Core target with load_relay_session_turns_ — 1 passed, including paged/full content equivalence after unloading the session.
• Same Core target with history_page_benchmark -- --ignored --nocapture — passed; measurements above.
• cargo test --locked -p openbitfun-services-integrations --no-default-features --features remote-connect --lib remote_connect::host_stream — 20 passed. Covers bounded reads, large-turn continuation, live updates, source gaps, epoch resets, eviction/backscroll and the existing Rust subscriber.
• node --test src/apps/mobile/harmonyos/tools/tests/host-stream.test.cjs — 10 passed.
• From src/apps/mobile/shared: ./gradlew :core-transport:jvmTest --tests '*HostStreamTest' — 19 passed, exercising the shared Android/iOS reader with JS-safe history/live cursors.
• pnpm run fmt:rs and git diff --check — passed.
• pnpm run mobile:architecture — existing failure. A pristine worktree of base faf5d4a41 produces exactly the same output: one sharedReachesPlatformTrees entry in ClientBuildContractTest.kt and three defaultArgsInFeatureApi entries. No baseline was loosened.

Remote scenario exercised: Remote Connect host/readers in local fixtures. HarmonyOS hardware validation could not run because hdc list targets returned empty. No live relay, SSH workspace, peer-device or detached-dispatch verification is claimed.

Reviewer Notes

• Historical backfill uses decreasing JS-safe sequences below the live sequence range, so it never advances the forward cursor. The existing per-session source gate orders backfill and live publication. Wire fields and persisted formats are unchanged; legacy clients retain their current path.
• Undo/import, source delivery gaps and reads across evicted cache ranges use an epoch fence. Backscroll can continue beyond the resident body budget without silently skipping records.
• File-name indices remain authoritative when a legacy catalog is absent or stale; unrelated turn bodies are not parsed. In-progress context overlay remains in the coordinator.
• The pending source batch is at most one persisted turn. A single very large turn still requires reading/projecting that file; this PR does not claim to eliminate that cost or cold-start connection setup.
• Based independently on upstream main, without bundling fix(mobile): let the host's transcript and catalog be the single mobile authority #3201. Both PRs touch the coordinator history reader; preserve both changes when resolving their merge overlap.

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 (no UI copy changes; owner verification guides updated).