reduce snapshot churn and speed up HistoryId lookups

[ConstantinHvber]

Implement fixes to address long-session TUI lag and history-state mismatch degradation.

• Fix 1: throttle non-forced history snapshot persistence (reduce full snapshot rewrite frequency to 5 secs during rapid UI/progress updates)
• Fix 2: add a lazy HistoryId→cell-index cache with dirty tracking + self-healing lookups; throttle “history-state mismatch” warnings; adjust tool-card/running-tool resolution to use the new lookup; add regression test to keep the index consistent under churn
• Fix startup panic: route the “Connecting MCP servers…” notice through the background helper (avoid inserting BackgroundEvent via “prelude” path) and remove the now-unused helper
• Cleanup: remove non-Linux cgroup stubs and adjust spawn/exec cfg blocks to eliminate macOS warnings

[Copilot]

Pull request overview

This PR addresses TUI performance degradation during long sessions by implementing a lazy HistoryId-to-cell-index cache with dirty tracking and self-healing lookups, throttling history snapshot persistence, and fixing a startup panic related to MCP server connection notices.

Key Changes:

• Added a lazy HistoryId→cell-index HashMap cache with dirty tracking and automatic rebuild/self-healing to speed up history cell lookups
• Throttled non-forced history snapshot persistence from 400ms to 5 seconds to reduce I/O during rapid UI updates
• Fixed startup panic by routing "Connecting MCP servers…" notice through the background helper instead of creating a special-case BackgroundEventCell

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
code-rs/tui/src/history_cell/mod.rs	Removed unused new_connecting_mcp_status() helper function
code-rs/tui/src/chatwidget/tools.rs	Reformatted tool resolution fallback chain for clarity
code-rs/tui/src/chatwidget/running_tools.rs	Changed resolve_entry_index to take &mut ChatWidget to enable cache lookups
code-rs/tui/src/chatwidget.rs	Implemented HistoryId index cache with dirty tracking, updated all insert/replace/remove operations to maintain index consistency, throttled snapshot persistence and mismatch warnings, added comprehensive test coverage
code-rs/core/src/spawn.rs	Removed non-Linux cgroup stub declaration to eliminate macOS warnings
code-rs/core/src/exec.rs	Refactored cgroup cleanup into platform-specific blocks to eliminate warnings
code-rs/core/src/cgroup.rs	Removed non-Linux cgroup stub functions, consolidated platform-specific code under cfg attributes

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