Step 4: WTL observer Bubble Tea msg bridge

[siisee11]

Summary

Implement Step 4 of the git-impact rollout by wiring WTL Observer lifecycle events into Bubble Tea messages, adding a minimal analysis progress model, covering bridge dispatch with tests, and moving the completed execution plan from active/ to completed/.

Milestones completed

• M1: Added Bubble Tea dependencies (bubbletea, bubbles, lipgloss) to go.mod/go.sum.
• M2: Added Observer->Bubble Tea bridge types and adapter in internal/gitimpact/tui_bridge.go.
• M3: Added minimal analysis progress model in internal/gitimpact/tui_model.go.
• M4: Added observer bridge dispatch tests in internal/gitimpact/tui_bridge_test.go.
• M5: Verified build and tests and finalized the plan artifact in docs/exec-plans/completed/step-4-of-10-implement-the-wtl-observer-bubble-tea-msg-bridge-for-git-impact-rea.md.

Key decisions

• Keep Step 4 scoped to Observer->TUI bridge and minimal state transitions; defer richer TUI UX to later steps.
• Model each required Observer lifecycle event as an explicit Bubble Tea Msg type to keep handling testable and unambiguous.
• Keep the adapter boundary in internal/gitimpact to avoid coupling WTL internals directly to TUI state logic.
• Introduce only minimal engine/observer/result scaffolding needed to compile and validate bridge behavior now.

Test plan

• go build ./...
• go test ./...