RFC: Extract DisplayState from IndicatifRenderer for testability

[evansenter]

Summary

Extract a DisplayState abstraction from IndicatifRenderer to separate state tracking from terminal rendering, enabling unit testing of display logic.

Problem

Currently, IndicatifRenderer mixes:

1. State tracking - subagent depth, current phase, tool execution status
2. Terminal rendering - indicatif progress bars, spinners, colored output

This makes unit testing difficult since we can't test state transitions without mocking terminal output.

Proposed Solution

Create a DisplayState struct that tracks:

• Current subagent nesting depth
• Active phase (decomposition, execution, synthesis)
• Tool execution status
• Token usage

The Renderer trait would consume DisplayState for actual rendering.

/// Terminal-agnostic state machine for display logic
pub struct DisplayState {
    pub subagent_depth: usize,
    pub current_phase: Option<String>,
    pub active_tools: Vec<ToolStatus>,
    pub token_usage: Option<TokenUsage>,
}

impl DisplayState {
    pub fn on_event(&mut self, update: &AgentUpdate) -> StateChange {
        match update.event_type.as_str() {
            "subagent_started" => {
                self.subagent_depth += 1;
                StateChange::SubagentEntered(...)
            }
            "subagent_completed" => {
                self.subagent_depth = self.subagent_depth.saturating_sub(1);
                StateChange::SubagentExited(...)
            }
            // ...
        }
    }
}

Benefits

1. Testable state logic - Can test DisplayState::on_event() without terminal
2. Swappable renderers - Same state, different backends (indicatif, ratatui, plain)
3. Clearer separation - State machine vs presentation

Test Cases to Enable

#[test]
fn test_subagent_depth_tracking() {
    let mut state = DisplayState::new();
    state.on_event(&subagent_started_event("deep_research"));
    assert_eq!(state.subagent_depth, 1);
    
    state.on_event(&subagent_completed_event("deep_research"));
    assert_eq!(state.subagent_depth, 0);
}

#[test]
fn test_nested_subagents() {
    let mut state = DisplayState::new();
    state.on_event(&subagent_started_event("developer"));
    state.on_event(&subagent_started_event("deep_research"));
    assert_eq!(state.subagent_depth, 2);
}

Scope

• Extract state tracking from gemicro-cli/src/display/indicatif.rs
• Create gemicro-cli/src/display/state.rs
• Update IndicatifRenderer to use DisplayState
• Add unit tests for state transitions

Context

Identified during PR #219 (Phase 2 - Subagent Access) when evaluating test coverage gaps. The subagent rendering logic is currently untestable.

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