feat(pulse): add timeline annotations and event markers with sliding cards

[ingvarch]

Description

Brings Timeline Annotations & Event Markers to Pulse, introducing a core capability inspired by cloud observability platforms (Grafana, Datadog) to terminal charts.

When monitoring metrics (CPU, latency, memory), raw spikes don't explain why something happened. This feature allows operators and developers to pin discrete engineering events (deployments, alerts, pod restarts, config changes) onto the time-series grid.

Key Changes

• Core Types & APIs:
  • Added Event struct (ID, Label, Glyph, Style, NoLine) and VisibleEvent (Col, Age, Event).
  • Added chart.AddEvent(ev) (pins at now), chart.AddEventAt(offset, ev) (pins at historical step), chart.VisibleEvents(), and chart. ClearEvents().
  • Added constructor option pulse.WithEvent(offset, ev).
  • Sliding Window Lifecycle & Memory Safety:
  • Tracks discrete steps synchronized with Push / PushSeries.
  • Dynamically calculates event age: age = current_step - event_step. Events scroll smoothly to the left as new data points arrive.
  • Automatically prunes events that scroll off the left edge (col < 0) to prevent memory leaks during long-running streaming sessions.
• Rendering Engine Integration:
  • Anchors marker glyphs (▼, 🚀, ⚡, ⚠️) at row 0.
  • Draws vertical dashed guidelines (┆) through empty space and shaded/tinted area fills without corrupting box-drawing line turns (╭, ╯, ─).
  • Handles multi-width Unicode runes and emojis (lipgloss.Width(glyph) > 1) in the terminal grid to prevent column drift.
  • Seamlessly supported across both ModeLines and ModeBraille.
• Companion UI:
  • Added chart.EventsBox() helper that returns a Lip Gloss bordered card listing active events with relative age tags ((now), (-14)).
• Interactive Demo & Documentation:
  • Updated examples/livedemo with interactive key e to drop live deployment markers in real time.
  • Created comprehensive guide in docs/timeline-events.md covering best practices, glyph selection, and code snippets.
  • Updated README.md features and documentation index.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement / Refactoring

Checklist

• I have read the CONTRIBUTING.md guide.
• My code adheres to the project's code style and conventions (gofmt, go vet).
• I have added tests that prove my fix is effective or that my feature works.
• All existing and new tests pass (go test -race ./...).
• I have updated any relevant documentation.