Introduce cross-platform shell core

[realmorrisliu]

Summary

• add pure Rust alan-shell-core and shell-core-ffi crates for portable shell workspace model, reducer, manifest, action, control, terminal profile, and settings summary semantics
• wire the macOS client through ShellCoreFFIAdapter for migrated manifest/reducer/action/control/profile/settings paths while keeping platform IO and runtime effects in Swift
• add Swift-exported parity fixtures, Rust fixture tests, adapter smoke tests, architecture guards, and OpenSpec validation fixes needed for the migration gate

Validation

• cargo fmt --all
• cargo test -p alan-shell-core -p alan-shell-core-ffi
• cargo clippy -p alan-shell-core -p alan-shell-core-ffi --all-targets -- -D warnings
• bash clients/apple/scripts/test-shell-core-ffi-adapter.sh
• bash clients/apple/scripts/test-shell-runtime-metadata.sh
• bash clients/apple/scripts/test-terminal-runtime-service.sh
• bash clients/apple/scripts/test-shell-split-model.sh
• bash clients/apple/scripts/test-shell-action-registry.sh
• bash clients/apple/scripts/test-shell-workspace-manifest.sh
• bash clients/apple/scripts/test-shell-settings-surface.sh
• bash clients/apple/scripts/test-terminal-account-dev-dry-run-smoke.sh
• bash clients/apple/scripts/check-shell-contracts.sh
• bash clients/apple/scripts/check-architecture-maintainability.sh
• openspec validate introduce-cross-platform-shell-core --type change --strict --json
• openspec validate --all --strict --json
• git diff --check

Note: test-shell-settings-surface.sh first hit a transient Clang module-cache rename error while several Swift scripts ran in parallel; a separate rerun passed.

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[chatgpt-codex-connector]

Route terminal.send_text through the host handler

When a control-file command uses terminal.send_text, ShellControlPlane.responseForPolledCommand tries socketServer.handleLocally before the host commandHandler; adding .terminalSendText here makes AlanShellLocalCommandExecutor consume the command and return a sendText side effect. However ShellControlPlane constructs the socket server with sideEffectHandler: { _ in }, so the side effect is dropped and the host path in ShellHostControlCommandHandling that actually writes to TerminalRuntimeRegistry is skipped. In that polled-command context, the response can report success while no bytes are delivered.

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