feat(pulse): add bidirectional zero-crossing fill and RX/TX traffic support

[ingvarch]

Description

Add bidirectional zero-crossing fill and first-class support for RX/TX network traffic and mirrored time-series metrics.

Key capabilities:

• Zero baseline rendering (WithZeroBaseline): explicit baseline ruling (─) and axis delimiter (├) anchoring coordinates at Y = 0.
• Bidirectional area fill: positive values fill downward to zero (┴), negative values fill upward to zero (┬), and intersections resolve cleanly (┼). Supported in both ModeLines and ModeBraille.
• Negative styling (WithNegativeStyle, WithSeriesNegativeStyle): distinct styles and tints for negative phase regions.
• Series inversion (WithSeriesInverted, WithInverted): visual negation for egress traffic (v -> -v) while preserving raw positive magnitudes in data storage and Last().
• Auto-symmetric range (WithSymmetric, SetSymmetric): automatically normalizes range bounds to [-limit, +limit] around zero, keeping the zero baseline centered vertically.
• Adaptive byte and throughput rate formatting (scale.FormatBytes, scale.FormatBytesRate, scale.FormatBytesRateAbs, scale.BytesRateFormatter): unit-adaptive formatting for network metrics and mirror Y-axes.
• Interactive Bubble Tea demo (examples/rxtxdemo) showcasing live network traffic with real-time hotkeys.

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.