fix(session): harden cross-surface persistence and ACP lifecycle

[limityan]

Summary

• make Core session mutation and persistence paths converge on the same per-session ownership boundary, including mode updates, turn start, restore migrations, metadata RMW, deletion, compression, active-session capacity, and maintenance drain
• make TUI mode changes non-blocking and recoverable: editing, resize, session switch, new session, and host commands remain usable; the source session cannot submit until persistence settles; exit supports wait-then-exit and explicit force-exit
• make missing persisted modes visible in TUI without expanding the Runtime SDK: CLI compares the pre/post restore summaries, explains the fallback, and preserves any startup prompt for explicit user confirmation
• prevent Desktop metadata writers and relay history imports from overwriting or publishing incomplete session facts by using owner-side field masks and a private pending/complete import marker before Core restore
• complete ACP create/load/close compensation, transition serialization, exact maintenance drain, stable ephemeral MCP ownership, and retryable close errors while preserving persisted history

Product and architecture boundaries

• no extension capability, HarmonyOS work, remote execution behavior, or new product surface is added
• no public mode-migration DTO, generic session-settings API, or parallel session owner is introduced; the fallback notice remains CLI-local
• existing metadata/state multi-file persistence is not presented as crash-atomic or cross-process transactional
• ACP transition ownership remains scoped to one stdio process; multi-process writes to one persisted session remain unsupported
• session/close only releases active ACP/Core/MCP resources and retains reloadable history
• the temporary decomposition plan is not part of this PR; only existing architecture documents are updated with implemented facts

End-to-end behavior

Path	Before	After
TUI mode save	UI could block or diverge from durable state	UI commits only after Core success; source-session input is preserved; failure is retryable
TUI exit during mode save	no reliable escape/reconciliation path	first exit waits and exits only on success; second exit leaves immediately and restores the last durable mode next time
missing persisted mode	fallback could run a startup prompt without notice	TUI shows old-to-new mode and requires explicit send
Desktop metadata writers	full stale snapshots could overwrite independent owners	explicit UI field masks merge under the owner lock
relay-imported history	partial local turns could be treated as complete	pending/complete plus exact count is checked before Core restore; failure stays retryable and fail-closed
ACP close/setup cleanup	partial ownership and retry semantics were ambiguous	ownership is retained until cleanup completes; stage and recovery action are returned
same-session persistence	mode, turn, compression, and deletion could interleave	one normalized session mutation/path-lock boundary serializes changed writers

Validation

Independent architecture, product-design, and reliability reviewers adversarially reviewed the final tree. All P0/P1/P2 findings and wording-level follow-ups were addressed before submission.

• cargo check --workspace
• cargo test -p bitfun-core - 1211 passed, 1 ignored
• cargo test -p bitfun-cli - 256 unit tests plus ACP stdio, exec, plugin, product-assembly, and PTY/ConPTY contracts passed
• cargo test -p bitfun-acp --lib - 101 passed
• cargo test -p bitfun-desktop - 110 passed
• cargo test -p bitfun-services-integrations --features mcp - 12 unit and 35 MCP contract tests passed
• pnpm --dir src/web-ui run test:run - 261 files / 1626 tests passed
• pnpm run lint:web - 0 errors; 12 unchanged warnings
• pnpm run type-check:web
• node scripts/check-core-boundaries.mjs
• git diff --check gcwing/main --

A prior full cargo test --workspace --no-fail-fast sweep exposed one unchanged Windows extended-path-prefix assertion outside this PR. The affected changed packages and MCP contracts pass in the final isolated runs above.