refactor(pulse): decompose monolithic core, optimize render allocations, and modularize test suite

[ingvarch]

Description

This PR refactors the core charting package following Go best practices, Single Responsibility Principle (SRP), and memory optimization guidelines:

1. Monolithic File Decomposition:

   • Decomposed pulse.go (1,305 lines) into 8 focused, single-responsibility modules:
     • pulse.go (318 lines): Core Model state, streaming window (Push, PushSeries), and Chart getters/setters.
     • view.go (115 lines): View() orchestrator, fmt.Stringer, and tick label calculation.
     • options.go (98 lines): Functional constructor options (With...).
     • event.go (150 lines): Timeline events, aging, pruning, EventsBox(), and shared pin/guideline rendering.
     • legend.go (37 lines): Multi-series legend presentation (Legend(), LegendBox()).
     • renderer.go (161 lines): Rendering contracts, coordinate math (spanOf, normOf, tickRowOf), and grid assembler.
     • renderer_lines.go (254 lines): Box-drawing curve engine, glyph masks, shaded and tinted fills.
     • renderer_braille.go (252 lines): Braille dot matrix and Catmull-Rom spline interpolation.
   • Every source file in the package is now strictly under 320 lines.

2. DRY Event Rendering:

   • Unified event marker badges (▼, 🚀, ⚡) and vertical guidelines (┆) into shared helpers (eventsByColumn, renderEventPin, renderEventGuideline).
   • Eliminated duplicated event styling logic across LinesRenderer and BrailleRenderer.

3. Performance & Memory Allocations:

   • Zero-copy Series Data in Render Loop: RenderContext now passes slice references directly (SeriesData [][]float64), eliminating redundant make([]float64) and copy() calls per series on every frame.
   • Contiguous Grid Backing Buffer: Refactored makeCells[T] to use a single continuous 1D buffer (make([]T, h*w)) with slice headers. Drops heap allocations by 76+ on every call to View().

4. Defensive Range Normalization & Styling Constants:

   • Automatically swaps inverted ranges (min > max) in New and SetRange.
   • Extracted DefaultEventColor (#7dcfff) to replace scattered color string literals.

5. Benchmark Suite:

   • Added pulse_bench_test.go covering View() across ModeLines, TintedFill, and ModeBraille, plus Push, LegendBox, and EventsBox with allocation tracking (b.ReportAllocs()).

6. Test Suite Modularization:

   • Split pulse_test.go (673 lines) into domain test suites:
     • event_test.go (132 lines): sliding window aging, off-screen pruning, EventsBox.
     • legend_test.go (39 lines): Legend() and LegendBox().
     • renderer_test.go (274 lines): grid lines, fill modes, tinted and solid area fills, line styles, label widths.
     • pulse_test.go (237 lines): core model buffer, multi-series, range normalization, and tick sorting.

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.