feat: add configurable line width, align LineStyle with MarkerStyle

[zao111222333]

This PR follows the same design direction as MarkerStyle and refactors LineStyle into a styling struct that supports both line type and width.

Previously, line styling was focused on pattern selection only. With this change, LineStyle becomes a more complete and extensible API for series lines and reference lines, making line configuration more consistent with the existing marker styling model.

Key changes:

• Introduce a LineStyle struct that stores both width and line type
• Reuse the shared size abstraction so line widths can be defined in pixels or world units
• Update series and reference-line rendering so custom line widths are actually applied
• Adjust builders and related APIs to keep line styling ergonomic and consistent

Overall, this makes line styling match the MarkerStyle approach more closely and provides a clearer foundation for future styling extensions.

[zao111222333]

commit-56277 adds anti-aliasing for shader-rendered lines.

I discovered this while working on a local PlotWidget implementation for the tiny-skia CPU backend, which I plan to submit in a follow-up PR. Compared with the canvas path, the existing shader path rendered diagonal lines with visibly hard edges.

This change replaces the previous hard-edged polyline-strip approach with a screen-space analytic AA path:

1. Each polyline segment is rendered as an independent quad instead of a single continuous triangle strip.
2. In the vertex shader, the segment is transformed into screen-space basis vectors:
   • tangent_px for the along-line direction
   • normal_px for the perpendicular direction
3. The generated quad is expanded by the true half line width plus a small AA fringe (LINE_AA_RADIUS_PX = 1.0).
4. The vertex shader passes local screen-space coordinates to the fragment shader:
   • local_x_px
   • local_y_px
   • half_width_px

The fragment shader then computes stroke coverage analytically:

• local_x_px is clamped to the segment extent to handle caps correctly.
• The pixel-space distance from the fragment to the ideal stroked segment is computed.
• That distance is converted into alpha with a soft ~1 px falloff.

let distance_to_stroke_px = length(vec2<f32>(cap_dx_px, in.local_y_px));
let edge_alpha = clamp(in.half_width_px + 0.5 - distance_to_stroke_px, 0.0, 1.0);