feature: draw a colored braided sleeve around a cable bundle (#503)

[VoltixSpark]

Refs #503

What

Adds an optional sleeve_color cable attribute that draws a colored braided sleeve/wrap around the conductor bundle, and adds a + Sleeve indicator with the color name to the cable header.

WireViz can already list a sleeve via additional_components (BOM only); this renders it on the diagram, which is useful for harnesses that use colored braided sleeving as a build and identification aid.

Screenshot

Example

cables:
  W1:
    category: bundle
    wirecount: 4
    gauge: 18 AWG
    length: 0.8
    colors: [BK, BK, RD, RD]
    sleeve_color: RD

Implementation

• wv_dataclasses.py: new Cable.sleeve_color (SingleColor), converted in __post_init__ like the other color fields.
• wv_graphviz.py: gv_sleeve_braid_band() builds a two-tone basket-weave band; gv_conductor_table() draws it across the top and bottom of the bundle with thin colored side rails when sleeve_color is set; the cable header gains a + Sleeve <color> indicator.
• docs/syntax.md and docs/CHANGELOG.md updated.

Composes with shield, works for both bundles and sheathed cables, and has no effect when unset. No new runtime dependencies.

Notes / open questions

• GraphViz HTML labels cannot draw a true diagonal braid, so the band is a two-tone basket-weave approximation (the sleeve color plus a darker shade). I am happy to adjust the visual treatment based on maintainer preference (for example, a plain tinted "tube" fill is an easy alternative).
• The band currently uses a fixed cell count; if you would prefer it to track the conductor-table width exactly, I can iterate.
• Formatted with black and isort per CONTRIBUTING. Generated example/tutorial outputs are intentionally not included.