feat(runtime): unify active-turn steering across clients - #1979

Summary

• Add a text-only active-turn steering operation to the stable Agent Runtime port, then route Core, Desktop, Dispatch, Embedded TUI, and Shared TUI through that single owner.
• Make ordinary Enter in the busy CLI/TUI steer the exact active turn while keeping slash-command and Shell-mode behavior unchanged. Rich drafts fail closed and remain editable.
• Reuse the GUI's existing PendingQueue owner: preserve full queued payloads during explicit drain and keep images or structured metadata queued instead of flattening them into text.
• Version the private Shared TUI IPC to v13, validate exact session/turn identities, and render optimistic steering inline with authoritative event deduplication.

Competitor alignment

• OpenCode 1882c33827cf0ce5c948b69ab5a87ed8f6790cf8: regular TUI submission uses the normal prompt path and Core defaults delivery to steer; App follow-up defaults to steering.
• Codex 2b5bdcf67547860f2e5c5a605009a70026796b2b: an ordinary user turn first attempts active-turn steering and only uses turn start/fallback when steering is not applicable.
• BitFun therefore keeps the native Enter flow and does not add a /steer command. This slice remains intentionally text-only; images, workspace references, and structured GUI metadata are preserved for regular turn submission.

Architecture

• AgentDialogTurnPort is the only steering boundary; the old Core compatibility bypass is removed.
• Core acceptance uses the existing per-session operation lock plus the authoritative active-turn store, and rejects stale or mismatched turns.
• Shared TUI admission stays private and connection-scoped; invalid runtime result kinds or identities close the connection rather than acknowledging an ambiguous mutation.
• GUI queue lifecycle and persistence remain owned by the existing PendingQueueManager; no second queue or delivery state machine is introduced.
• No Relay changes are included.

Verification

Passed locally:

• cargo check --workspace
• Core feature checks for workspace-runtime, remote-workspace, ssh-remote, and --no-default-features
• Runtime Ports steering contract: 1 test
• Agent Runtime steering: 3 tests
• Shared TUI IPC steering: 3 tests
• Core steering: 5 tests
• CLI steering: 5 tests
• cargo check -p bitfun-desktop
• Web lint and type-check (lint has 2 pre-existing warnings)
• PendingQueue focused tests: 2 tests
• i18n audit and Core boundary checks

Environment notes:

• The full Web suite passed 376/377 files and 2570/2571 tests locally. The remaining unrelated Remote Connect source-contract test hard-matches LF while this Windows checkout materializes the unchanged source as CRLF.
• cargo test -p bitfun-desktop was attempted after the check passed but the machine ran out of D: drive space while compiling third-party dependencies; no test assertion or PR compiler error was reached.