refactor: extract delegates state

[zatevakhin]

scope

• extract private DelegatesState from exact integrated ComposerState base e1f984aa26ffa1dca3eebef7041324e41a659873
• move exactly 14 delegate-owned fields and defaults behind the single App::delegates owner
• move popup filtering/selection/navigation, lifecycle reconciliation, child state/stat/message accumulation, pending tool-call linkage, and root clearing into the owner
• keep ACP parsing/event matching, commands, session/profile/model coordination, status updates, and render-cache invalidation at existing adapter/coordinator boundaries
• preserve root/child reset asymmetry, stale cursor/filter/visible-row behavior, unordered map behavior, sibling parent staging, pending elicitation restoration, and duplicate child assistant suppression

boundary proof

• private module declarations: 1
• App delegate-owner fields: 1
• DelegatesState fields: 14
• direct App fields: 53 -> 40
• old delegate flat fields on App: 0
• delegate_model_preferences references in owner: 0
• forbidden owner dependencies (App/Command/ACP/protocol/UI/runtime/config/channels/persistence): 0
• public export, alias, getter facade, or Deref: 0
• phase 11 compatibility-forward tags: 4
• changed files: exactly the closed 10-file allowlist
• protected AGENTS.md, CLAUDE.md, ignored REFACTOR.md, and stuff/history/: unchanged/uncommitted

metrics

• diff: 10 files, +996/-394
• Rust files: 53 -> 54
• physical Rust LOC: 45,097 -> 45,699
• test declarations: 860 -> 872
• App implementation files: 4

validation

• cargo fmt --all -- --check
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets --all-features
• cargo test --all-targets --all-features (872 passed, 0 failed, 0 ignored)
• git diff --check

pending

Integration and manual TUI smoke are intentionally not claimed. ChatState, RenderState, and Phase 5 remain pending and are not part of this slice. This PR must not be merged as part of this handoff.